Officials in Texas are putting the initial death toll as high as 15 following a massive fertiliser plant explosion in the small town of West, but fear dozens more are dead.
Local authorities say more than 160 people have been injured in the blast, which flattened nearby homes and businesses.
The town's mayor, who likened the blast to "a nuclear bomb", says as many as seven firefighters and one police officer are unaccounted for.
The missing authorities were on the scene, attending to a massive fire at the plant when the explosion occurred.
A five-block area of the town was levelled and there are fears people are trapped or dead inside about 70 collapsed buildings.
Such was the power of the blast, the US Geological Survey says it registered as a magnitude-2.1 event even though the nearest seismic sensor is about 40 kilometres away.
Waco police sergeant Patrick Swanton told a press conference on Thursday night (AEST) that the death toll "is estimated anywhere from five to 15 at this point", but added that those figures could change.
West is home to around 2,700 people and is about 130km south of Dallas and 32km north of Waco.
Emergency teams have worked through the night searching buildings in the hope of finding more survivors.
An apartment complex and a nursing home were destroyed and local residents flooded into emergency shelters.
Local police Sergeant Patrick Swanton says evacuations are continuing due to the ongoing danger.
"The fires are under control but still burning," he said.
"My commander, who is on the ground there, is telling me that he is seeing extreme devastation in homes, in some of the businesses."
Much of the centre of town has been evacuated as fires are still smouldering at the plant, sending a toxic plume across the area.
The cause is not immediately known but it is believed to have been an anhydrous ammonia explosion.
Officials are expected to give a further briefing on details of the emergency later tonight (Australian time).
The explosion comes as the US is on edge as it pieces through deadly explosions that hit the Boston marathon earlier this week.
Look back at how the day's events unfolded. All times AEST
West residents are waking to scenes of destruction in their small town.
Stephanie Ando, a reporter with KBTX Media in the US, described the scene to ABC News 24 just a little while ago.
"Right now we're just starting to see light today. We've actually been here, me and my cameraman, have been here for about seven hours now," she said.
"I can tell you that when we first got here the area near the scene was completely blocked off.
"The only people they were letting in and out were emergency responders and you can imagine they were just focussing on treating the people who were injured at first.
"It's a very small community so it was very obvious that there were emergency vehicles going up and down the highway taking people to the hospitals for hours.
"You could smell the chemicals in the air and anyone that you talked to on the side of the road, people at gas stations, you know, everyone in the community knew someone who was affected or could tell their story about the moment that the explosion happened.
"Everybody felt it for miles away."
West local Paul Lannuier has uploaded footage of himself travelling through the town shortly after the explosion. He shows scenes of utter devastation from the town:
The mayor of West, Tommy Muska, gives an update on the situation, saying the nursing home near the fertiliser plant has been fully evacuated:
7.30's Leigh Sales recaps today's events in West and speaks to Waco police about the situation:
North America correspondent Brendan Trembath speaks to PM about the number of injured:
"Authorities are refraining from releasing a death toll at this stage. There is talk about hundreds injured. One of the local hospitals in the area just up the road is Waco, about 30 kilometres away. There there's a hospital called Hillcrest and they've treated more than 100 there."
"Earlier I watched ambulances and police cars leave that scene and a long line, so there are many, many people who have been injured."
West mayor Tommy Muska tells CNN there is no indication the explosion is anything but an accident.
We now have video of Sergeant Swanton's media conference from earlier.
Professor Priyan Mendis is an explosives expert with the Department of Infrastructure Engineering at the University of Melbourne, where he studies the effect of explosions on infrastructure.
He says he is very surprised at the number of casualties because ammonium nitrate is normally not very dangerous.
"Initial reports are showing it is anhydrous type of ammonium," he told 774 ABC Melbourne.
"Anhydrous means it is without water and dry... it's a gas.
"It has to really combine with dust or other small particles for it to explode, it won't just explode."
Professor Mendis says it is likely the fire caused the explosion.
The burning remains of the fertiliser plant.
When asked about the cause of the explosion, Sergeant Swanton said:
"At this point we don't know."
He says he cannot rule out criminal activity.
Sergeant Swanton says there has been overwhelming support offered from surrounding communities.
"I will tell you there has been a tremendous amount of resources and outpouring from community, not only here in West.
"Those of you that don't know West, it's about 2,800 people. They are a very close-knit community here.
"They have relied heavily on each other tonight for the support they have got."
The media conference is being held in an auction barn - cows can be heard in the background.
Local police Sergeant Patrick Swanton says evacuations are continuing due to the ongoing danger.
"The fires are under control but still burning.
"I just talked to my commander who is on the ground there. He is telling me that he is seeing extreme devastation in homes, in some of the businesses.
"They are still getting injured folks out and they are evacuating people from their homes."
Sergeant Swanton again confirms there have been fatalities and that the number will become clearer as day breaks.
"I can confirm there may be firefighters that are unaccounted for and potentially a law enforcement officer as well.
"We are still trying to determine that.
"Obviously they were there on scene directing traffic and fighting the fire and helping with the evacuation.
"Again, we don't know a hard number of the fatality count.
"As soon as we can get that, we will get that information to you."
Officials are holding another update for the media.
Chris Sadeghi, a local journalist, paints a picture of the town.
"It's a very small town.
"Even though it's so small it's a town that a lot of people in Texas know about.
"They're known for making some of the best kolaches.
"It has a spot in the heart of a lot of Texans."
Kolache is a type of pastry that holds dollop of fruit in the middle.
The Texas department of public safety says there is concern that one unexploded tank is venting gas from fertiliser plant.
Depending on wind direction and other factors, people living to the south and west of the plant may need to be evacuated.
People north of the plant are being asked to stay in their homes.
Here is a selection of images taken by photographers Rod Aydelotte and Kirsten Crow for the local Waco Tribune-Herald newspaper.
More photos can be viewed here.
Dr Bradford Holland is a head and neck surgeon in Waco. He says medical staff are being told to rest up for a busy day tomorrow.
#Westexplosion Medical staff being thinned now. Told to get rest and prepare to care for a PACKED hospital tomorrow @Hillcrest_Waco
A White House official says the Obama administration was aware of the situation and monitoring the local and state response through the Federal Emergency Management Agency (FEMA).
Jason Shelton, 33, a father of two who lives less than a 1.6km from the plant, said he heard fire trucks heading toward the facility five minutes before the explosion.
He said he felt the concussion from the blast as he stood on his front porch.
"My windows started rattling and my kids screaming.
"The screen door hit me in the forehead... and all the screens blew off my windows."
Here is an aerial view of the plant's location in West.
Map: Fertiliser explosion in West, Texas
A severe thunderstorm warning has been issued for areas of northern Texas and the area around West could be affected around sunrise.
David Holley-Gates lives in Abbott, Texas, near West.
He says he is close with the people of West and has started a campaign to help them.
Hi David, I'm from ABC in Australia. Do u know people in West affected by explosion and can u explain what ur doing to help?
I'm trying to get stuff together to take down there. I'm close with the people down in West and I'm reaching out to get help.
Hav people started bringing donations to your house? What are you receiving? How hav the people you know in West been affectd?
yes to my house or to my highschool. And we will take absolutely anything. Houses are gone. People are hurt killed.
A team from the US Chemical Safety Board is heading to West to begin an investigation.
The US Geological Survey says the explosion registered as a magnitude 2.1 event even though the nearest seismic sensor is about 40 kilometres away.
Here is video of the press conference given a short while ago by Texas department of public safety official DL Wilson:
The worst industrial disaster in US history occurred in Texas on April 16, 1947.
French ship SS Grandcamp was docked in Texas City when a fire broke out on board.
About 2,300 tonnes of ammonium nitrate blew up, setting off a chain reaction of explosions and fires that killed more than 580 people.
Glenn Robinson, chief executive of Hillcrest Hospital in Waco, says the hospital is treating more than 100 people injured in the explosion.
But he says the majority of those people have minor injuries.
He says about 16 of them will need to stay in hospital overnight.
Stunning images of the blast continue to come in on social media:
A spokesman from the Texas department of public safety is giving a briefing to the media.
Sheriff DL Wilson says more than 100 people are injured, adding there are some fatalities but no numbers have been confirmed.
The spokesman says between 50 and 75 houses are damaged.
He says an apartment block of around 50 units was a "complete skeleton".
"All the injured right now have been taken care of. We're going to go back in and do another house to house search.
"We're going to worry about people right now, not property. We want people to be safe."
Sheriff Wilson says authorities do not need any more help.
"We are overflowing with help and we do not need any more help."
He says he walked through the explosion area and described it as like a war zone.
"I was there, I walked through the blast area, I searched some houses, it was just like Iraq."
West mayor Tommy Muska says six or seven firefighters were in the fertiliser plant at the time of the explosion and have not been accounted for.
Our thoughts are with emergency responders and medical staff working the West, TX disaster.
— Boston EMS (@BOSTON_EMS)
Crystal Anthony, who witnessed the explosion but has since been evacuated, told CNN the community is pulling together.
"We are a very close-knit community and that's why we all have pulled together."
She said the damage she saw was devastating.
Eerily, the explosion comes two days before the 20th anniversary of a fire in Waco that engulfed a compound inhabited by David Koresh and his followers in the Branch Davidian sect, ending a siege by federal agents.
Some 82 members of the sect and four federal agents died in the incident.
There is no immediate official word on what sparked the explosion as emergency personnel help victims and douse the flames.
McLennan County Sheriff Parnell McNamara said there was a lot of devastation.
"I've never seen anything like this.
"It looks like a war zone with all the debris."
West mayor Tommy Muska says he believes around 60 buildings have been levelled.
Speaking to reporters, Mr Muska, who is also a firefighter, could not confirm reports that some firefighters had been killed fighting the blaze.
He said it was a very dangerous situation for them due to the possibility of a second explosion.
The mayor says he has no confirmation on a death toll.
"There's a lot of people who won't be here tomorrow."
The chief executive of the Hillcrest Hospital in nearby Waco says an additional 250 to 300 additional staff have been called in to help.
Local journalist Paul Gately told ABC News 24 that the county sheriff had told him "very many" people were injured:
A school and a nursing home are reported to be among the buildings on fire.
Waco's Assistant Fire Chief Don Yeager earlier told the AFP news agency that the explosion had been caused by anhydrous ammonia.
"An explosion has ignited adjacent structures to the plant," he said, adding that firefighters from neighbouring areas were being rushed to the scene.
He said he had no confirmation of any deaths.
Here is stunning video of the explosion captured by a man who was in his car with his family some distance from the plant:
Local television station KWTX news is reporting that at least 60 people have been killed in the explosion.
The report could not be immediately verified by sources, but KWTX cited West EMS director George Smith as confirming the death toll.
Texas governor Rick Perry has issued a statement saying state emergency services have been mobilised to help their local counterparts.
"Our thoughts and prayers are with the people of West, and the first responders on the scene," he said.
The Waco Tribune newspaper reported that Bill Bohannan, who was visiting his parents at their house in West, witnessed the explosion, which happened during a fire at the plant:
"I said, 'This thing is going to blow' ... and I told my mom and dad to get in the car.
"I was standing next to my car with my fiancee, waiting for my parents to come out and [the plant] exploded. It knocked us into the car... Every house within about four blocks is blown apart."
Photos posted on social media sites showed what appeared to be a large cloud of smoke rising over the site of the blast, and a makeshift medical treatment centre set up on a local football field.
There are reports of a massive explosion at a fertiliser plant in the town of West in Texas.
The blast, apparently preceded by a fire at the plant, was reported about 8pm (local time) in the town of some 2,700 people about 130km south of Dallas and 32km north of Waco.