Dozens of hostages have reportedly been killed in an Algerian special forces raid to free them from Islamist militants who seized control of a BP gas facility in the country's remote south.
Scores of Algerians and up to 41 foreigners - including Americans, British, French, Japanese and Norwegian nationals - were taken hostage by Islamic militants at the facility near In Amenas yesterday.
This morning the Algerian military reportedly moved against the militants, said to be affiliated to the Khaled Abu al-Abbas Brigade, a shadowy group linked to Al Qaeda and led by former Al Qaeda in the Islamic Maghreb leader Mokhtar Belmokhtar.
The Reuters news agency is reporting that 30 hostages have been killed, including seven foreigners.
Citing an "Algerian security source", Reuters said the dead hostages included eight Algerians, two Japanese, two Britons and one French national.
Reuters said 11 militants had been killed. Only two of them were Algerians. The others were reported to be three Egyptians, two Tunisians, two Libyans, a Malian and a Frenchman.
Reports say the military operation is now over.
Algerian communications minister Mohamed Said said a number of kidnappers had been "neutralised" as they tried to flee, in the first official comment on the operation, but admitted that "some" hostages were killed or wounded.
Earlier, one of the militants said "warplanes and ground units" had begun an operation "to take the complex by force," and threatened to "kill all the hostages if the Algerian forces succeed in entering the complex."
He said 34 hostages and 15 kidnappers were killed in an army air strike, a claim that could not be verified.
What I can tell you is that the state Algerian media has reported four foreign hostages and six Algerian hostages have been freed by an Algerian army operation.
But the newsagency reports are also quoting Algerian sources saying that six foreign hostages were in fact killed during the operation and 25 escaped. So it's not really clear.
There is one definite hostage who is free, we're not quite sure how.
He is an Irishman called Stephen McFaul and we know he is free because he got in touch with his family and the family of course are very emotional about that and saying it must have been the luck of the Irish that he is free, but he is the only one we know about for sure.
Europe correspondent Mary Gearin
The gunmen said their attack was in retaliation for Algiers supporting French air strikes in Mali, and demanded that 100 radical Islamists held in Algeria be released and sent to northern Mali in exchange for the hostages.
Foreign governments voiced growing concern about the rescue operation which a foreign diplomat in Algiers said "did not go too well for the hostages."
The Algerian army appears to have launched its operation before informing Britain and the United States.
British prime minister David Cameron postponed a major EU speech planned for later today because of the crisis and warned that Britain should brace itself for "the possibility of bad news."
British foreign secretary William Hague has cut short his visit to Australia to deal with the crisis.
US secretary of state Hillary Clinton said she hoped there had been minimal loss of life:
"When you deal with these relentless terrorists, life is not in any way precious to them," she said.
I think the Algerian forces ... are a good strong army, well-equipped, well-trained, [but] they lack the kind of sophistication that British, American, Australian, French forces might have to enable them to carry out a kind of precision operation against hostage-takers.
So it wouldn't surprise me if we saw at the end of the day we did see perhaps the hostage-takers killed, but also some hostages as well.
Former British commander in Afghanistan Richard Kemp
"But when this incident is finally over we know we face a continuing ongoing problem... and we're going to do everything we can to work together to confront and disrupt Al Qaeda in the Islamic Maghreb.
Meanwhile heavy fighting is continuing in Mali, where French troops have joined the Malian army to stop the rapid advance by Islamist forces.
France now has 1,400 troops on the ground in Mali, and combat is underway against the rebels that it first began targeting from the air last week.
French president Francois Hollande said the hostage crisis in Algeria showed that French intervention in Mali was justified.
African troops are also due to join the fighting, with reports saying that Mali's eastern neighbour Niger has a convoy of armoured, fuel tankers and ambulances positioned on the border along with about 200 soldiers.
Earlier, the gunmen allowed some of the hostages to speak to the media to put pressure on Algerian forces not to storm the compound
An unidentified hostage who spoke to France 24 television said prisoners were being forced to wear explosive belts.
The hostage said their captors were heavily armed and had threatened to blow up the base if the Algerian army tried to storm it.
Another hostage, identified as a British man, spoke to Al Jazeera television and called on the Algerian army to withdraw from the area to avoid casualties.
"There are around 150 Algerian hostages. We say to everybody that negotiations is a sign of strength and will spare many any loss of life," he said.
At the moment, there is no indication that any Australians are caught up in the hostage crisis.
Defence Minister Stephen Smith says the ongoing conflict in Mali and Algeria could mean a more direct role for Australia, although it is unlikely that would include military involvement.
"As ever Australia stands ready and willing to contemplate that," he said.
"We're not considering any military engagement ourselves.
"It's not operationally our part of the world, but as a member of the Security Council we will be down the track looking to see whether Australia needs to make or it's appropriate to make some form of contribution."
Meanwhile, the attack in Algeria has not stopped stop France from pressing on with its campaign in Mali.