Adjoining residential buildings in New York's East Harlem neighbourhood have exploded and collapsed, killing at least five people and injuring dozens.
The blast sparked a fire at what remained of the five-storey buildings, engulfing the area near 116th Street and Park Avenue in thick smoke.
Dozens of fire crews battled the blaze amid fears that nearby structures might collapse.
Piles of debris including bricks and lumber were strewn over nearby footpaths.
New York mayor Bill de Blasio said it appeared the explosion was caused by a gas leak.
He said energy company Con Edison received a call alerting it to a possible gas leak at 9:13am (local time), about 15 minutes before the blast.
Local residents also spoke of smelling gas in the area.
Mr de Blasio said a number of people remained unaccounted for and a thorough search of the rubble would be made once the fire was extinguished.
He cautioned that some of those missing could simply be safe elsewhere and out of contact.
"This is a tragedy of the worst kind, because there was no indication in time to save people," he said.
"There is a tremendous amount of anxiety, but suffice it to say that every effort is being expended to locate each and every one of these (missing) individuals."
Map: Harlem explosion
Mr de Blasio said there was a "very heavy impact" on buildings that surrounded the two that collapsed, and that 250 firefighters attended the scene.
Witness Robert Santiago told CBS Television that he was sleeping when the explosion shook his bed and the floor.
"I thought the world was coming to an end, an earthquake or something like that. Terrible," he said.
Another witness who lived across the street told the channel he saw the building on fire and that his relatives living further away also felt the blast.
"I saw a lady running with no shoes on. It was crazy. It was like a war zone," he said.
"First of all, I thought it was an earthquake. I got calls from my family who felt it too and that was all the way up town. Man, it was crazy."
The Metropolitan Transportation Authority suspended all train services in and out of Grand Central Terminal for several hours due to the explosion next to its tracks.
Once again, we have been shown what our first responders are capable of under extraordinarily difficult circumstances. #eastharlemexplosion
— Bill de Blasio (@BilldeBlasio) March 12, 2014