Thirty hostages, including two Britons, were reported killed when Algerian forces defied international pleas for caution and stormed a BP gas field in the Sahara.
And today the UK Foreign Office said the crisis "remains ongoing".
David Cameron told MPs today that the Algerian government had not warned him it was planning to take military action .
He that the number of British citizens at risk in the terror attack has now been "quite significantly reduced" after reports last night that around 30 British citizens were believed to be "at risk".
Speaking in Australia today, Mr Hague told Sky News: "This remains a fluid and evolving situation and many details are still unclear, but the responsibility for the tragic events of the last two days squarely rests with terrorists who chose to attack innocent workers, murdering some and holding others hostage.
"Our priority remains at the moment to identify exactly what has happened to each British national caught up in this incident and, indeed, to help other countries determine what has happened to their nationals."
A US plane landed today at an airport near the desert gas plant to evacuate Americans caught up in the crisis, a local source told Reuters.
Eleven or more Islamist militants were said to have died when Algerian special forces, helicopters and – according to one account – warplanes launched a series of attacks on the sprawling complex seized by an Islamist group linked to al-Qa'ida on Wednesday. Twenty Britons remained unaccounted for, British sources said.
David Cameron said earlier that Britain “should be prepared for the possibility of further… very difficult news” following the death of a British oil worker in the initial Islamist attack. He postponed a speech he was due to give in Amsterdam today on Britain's future in Europe, saying he “simply cannot be away” until the crisis is resolved.
For most of the day utter confusion surrounded the Algerian military operation. Tonight there were reports that some of the raiders were still defying the Algerian forces inside the gas extraction plant. There were fears that the death toll might rise. An unknown number of foreign captives and 600 Algerians were reported to have been freed or to have escaped from their captors during the assault. Algerian security sources said tonight that 30 hostages had died in the battle, including eight Algerians, two Britons, two Japanese and one Frenchman. Algerian state television said earlier that two Britons and two Filipinos were among the dead.
Stephen McFaul, 36, from Belfast, was one of the lucky ones who got out. Tonight he telephoned his family to say he was safe. One of the freed Algerian hostages told the French newspaper Le Monde tonight that the Islamist raiders were a multinational group from a number of countries – including a man who spoke English with a perfect British accent. One of the dead militants was said to be a French national.
The Algerian Communications Minister, Mohamed Said Belaid, claimed the operation was a success. “A large number of hostages were freed and a large number of terrorists were eliminated and we regret the few dead and wounded,” he said.
According to one report, seven hostages were held for many hours after the main assault by a die-hard group of militants who resisted in part of the gas plant near the town of In Amenas, near the Libyan border. Radio France Internationale reported from Algiers that the raiders had made four attempts to flee this morning. A convoy of buses and trucks was finally attacked from the air by the Algerian military. The Algerians had told the raiders that they could leave the country without their hostages but the kidnappers had attempted to take some foreigners with them. The conclusion to the siege has strained relations between Britain and Algeria, with Mr Cameron admitting he only learnt the intervention was under way when he rang his opposite number, Abdelmalek Sellal, at 11.30am. He had stressed Britain wanted to be warned before any military operation. Mr Sellal said events were changing so quickly that his forces had been forced to act.
The Algerian assault came as French operations against Islamist rebels in neighbouring Mali entered a seventh day, with warplanes and helicopters continuing to pound targets in the centre and north of the country. Despite pledges of support from a number of European nations at a meeting of EU foreign ministers, none is sending troops. A French captive contacted by the newspaper Sud Ouest said the raiders had strapped explosives to some hostages. It appeared that the 20 or 30-strong gang of kidnappers had difficulty in controlling so many captives. There were reports during the day of small groups of Algerian and foreign workers escaping.
The unnamed Algerian captive who spoke to Le Monde tonight said that the raiders “seemed to know the site well and knew exactly what they were looking for.… They searched the living quarters for foreigners and told muslims that they were in no danger.”
The capture of the Algerian gas site was claimed by the Islamist splinter group, Mouwaghina Bi Dam (“Those Who Sign in Blood”) as a retaliation for French intervention in Mali. But Western governments suggested that the operation must have been planned for many weeks. The French President François Hollande said the hostage-taking in Algeria had proved France's argument that its armed intervention in Mali was about far more than the fate of the Malian government.
The raid on the BP complex is believed to have been inspired – and possibly led – by a veteran al-Qa'ida leader in North Africa, Mokhtar Belmokhtar. It was unclear tonight whether he was amongst the dead.
Western governments are believed to have urged the Algerian authorities – in vain – to take a softly-softly approach. Algiers has a reputation for taking a violent line with hostage-taking.
'Just like the riots in Belfast'
Stephen McFaul, the Belfast man who escaped from the hostage-takers, was laughing and joking as gunmen took over the plant, his family said tonight.
Stephen stayed in contact by mobile phone with his brother Brian, who said he could hear gunfire in the background. "It's like riots in west Belfast," said Stephen, who was discovered by terrorists hiding under a bed.
"When news came through that Stevie was free I told mum and dad and they hugged each other, shouting at the top of their voices," said Brian.