The fate of a number of British hostages held in a desert gas complex remains unknown in the wake of an Algerian military attempt to free them.
They were among foreign nationals held near In Amenas who the Algerian forces attempted to free from militants.
On Friday, a day after the operation began, the Foreign Office said "the terrorist incident... is ongoing".
A Briton was confirmed dead on Wednesday and David Cameron has warned of further possible "bad news".
The state-run APS news agency cited local officials as saying another two Britons and two Filipinos were killed on Thursday.
The prime minister postponed a major speech on Europe scheduled for Friday, and Foreign Secretary William Hague has cut short a trip to Australia to return to the UK.
The BBC's Nick Robinson said sources had told him officials were prepared for "multiple" British casualties.
Mr Hague said he condemned "utterly the wanton act of terrorism", and said his priority was to "identify exactly what has happened to each British national caught up in this incident."
He added in a tweet: "Primary responsibility for tragic events in Algeria rest with terrorists who murdered some and held others hostage".
A Foreign Office spokesman said he could not provide further information but added: "Our priority will remain the safety of British nationals and their co-workers. We cannot provide any details that might endanger their lives. But we are working round-the-clock to resolve this crisis."
A Foreign Office team in Algiers had not yet been given permission by Algerian officials to visit the site, BBC correspondent Gordon Corera reports.
The full details of the operation launched by the Algerian military on Thursday, a day after the gas plant workers were taken hostage, are still emerging.
But reports suggest Algerian forces used helicopter gunships to attack the compound as hostages were being moved in vehicles around the site.
As many as 600 Algerians and four foreigners, including two Scots, were freed but others died, the state news agency reported.
'Still being searched'
Earlier the British government said it had been told the military operation was over, with the gas plant still being searched to see if people were hiding or if there were still groups of hostage-takers who had not yet been located.
Mr Cameron chaired another meeting of the government's emergency response committee, Cobra, for about 45 minutes on Friday morning. He will make a statement to Parliament at about 11:00 GMT.
Mr Cameron said on Thursday night that the country faced "a very bad situation".
"A number of British citizens have been taken hostage. Already we know of one who has died," Mr Cameron said.
"It is a very dangerous... a very fluid situation and I think we have to prepare ourselves for the possibility of bad news ahead."
Later, Foreign Office minister Alistair Burt said: "I am afraid we should be under no illusion that there will be some bad and distressing news to follow from this terrorist attack."
BBC political editor Nick Robinson said: "You saw in [Mr Cameron's] face, you heard in his voice, you could tell from the words just how anxious they are in Whitehall about further casualties.
"I am being told that they are prepared for what are being called multiple casualties. Early on in this siege there was talk, all of it unconfirmed, of as many as 35 dead from all nationalities."
Our editor added that some British hostages were known to be safe, but he had been told up to 20 were still to be accounted for.
"Some may still be missing, some... could still be hiding, others could be injured and not be properly identified, but sadly, too, there may be others who are dead."
Mr Cameron had been due to make a long-awaited speech on the UK's future relationship with Europe in the Netherlands on Friday, but made the decision to postpone his speech after a meeting of Cobra and a phone call with the Algerian prime minister.
No 10 said Mr Cameron had not been informed of the operation in advance and learned of it only when he had phoned his Algerian counterpart at 11:00 GMT on Thursday.
Shadow foreign secretary Douglas Alexander said the reports of the "grave and tragic events" were "deeply concerning".
Among the Britons freed were:
The Algerian communications minister, Mohamed Said Belaid, said the military operation had been "successful in neutralising a large number of terrorists and freeing a large number of hostages".
"But unfortunately, and we are sorry to say, there were some deaths and injuries. We do not yet have a definitive figure."
Algerian Interior Minister Daho Ould Kablia said the kidnappers were Algerian and operating under orders from Mokhtar Belmokhtar, who was a senior commander of al-Qaeda in the Islamic Maghreb (AQIM) until late last year.
The militants had said they were holding the hostages in retaliation for French military intervention in neighbouring Mali.
In a statement, the gas facility's part operator, BP, said its focus was "100% on the safety and welfare" of the workers and their families.
"We are now beginning a staged and planned reduction in non-essential workforce," it said.