Governments scrambled Sunday to track down missing nationals after the bloody end to a gas plant siege in the Sahara that saw Islamists kill at least 23 foreigners and Algerians, mostly hostages, as Algiers feared the toll may rise.

"I fear that it may be revised upward," Communications Minister Mohamed Said told public Channel 3 radio of the number of dead a day after Special Forces stormed the remote desert facility to end a crisis that saw seven foreigners killed by their captors in the final moments.

British Prime Minister David Cameron said in a statement on Sunday he had spoken to his Algerian counterpart Abdelmalek Sellal, "and it is now clear that this appalling terrorist incident in Algeria is now over.

"Tragically, we now know that three British nationals have been killed, and a further three are believed to be dead. And also a further British resident is also believed to be dead."

Colombian President Juan Manuel Santos said on Saturday that a Colombian man who lived in London with his family had been killed. He was named as BP employee Carlos Estrada.

Japanese engineering firm JGC Corp said 10 of its Japanese and seven of its foreign workers remained unaccounted for.

JGC confirmed the safety of 61 of its 78 workers at the In Amenas facility that was stormed at dawn Wednesday by militants from "Signatories in Blood," a group demanding an end to French military intervention in Mali.

"But the safety of the remaining 10 Japanese and seven foreign workers is yet to be confirmed," a JGC spokesman said in Tokyo.

Kuala Lumpur said JGC had told it one of two Malaysians still unaccounted for is dead whilst the fate of the other was unknown.

Norway's Statoil, which operates the gas plant alongside Britain's BP and Sonatrach of Algeria, said the situation remains "unresolved" for five Statoil employees.

"We will, and we must, keep hoping for more positive news from Algeria. However, we must be prepared to deal with bad news in the next few days," Statoil CEO Helge Lund said.

Thirty-two kidnappers were also killed in the 72-hour stand-off, and the army freed "685 Algerian workers and 107 foreigners," Algeria's interior ministry said Saturday.

Among the dead were an unknown number of foreigners.

Relatives of Kenneth Whiteside, 59, from Glenrothes in Scotland, were "devastated" after hearing that an Algerian co-worker claimed to have seen him being shot but dying bravely with a smile, Britain's Mail on Sunday reported.

The mother of survivor Stephen McFaul, 36, from Belfast, told the Sunday Mirror her son will "have nightmares for the rest of his life after the things he saw".

Forced to wear explosives, he fled when the hostage-takers' convoy he was in came under fire on Thursday.

In Saturday's final assault, "the Algerian army took out 11 terrorists, and the terrorist group killed seven foreign hostages," state television said, without giving a breakdown.

A security official said it was believed the foreigners were executed "in retaliation."

The militants, whose leader is Algerian Mokhtar Belmokhtar, a former Al-Qaeda commander, first killed a Briton and an Algerian on a bus on Wednesday before taking hundreds of workers hostage when they overran the gas plant.

Most of the hostages were freed on Thursday when Algerian forces launched a first rescue operation which was widely condemned as hasty.

But US President Barack Obama and his French counterpart Francois Hollande said responsibility for the deaths lay with the "terrorists."

"The blame for this tragedy rests with the terrorists who carried it out, and the United States condemns their actions in the strongest possible terms," Obama said in a statement.

"In the coming days, we will remain in close touch with the government of Algeria to gain a fuller understanding of what took place so that we can work together to prevent tragedies like this," Obama added.

At least one American had already been confirmed dead before Saturday's final assault.

Hollande called Algiers' response "the most appropriate" given it was dealing with "coldly determined terrorists ready to kill their hostages."

Monitoring group IntelCenter said the hostage-taking was the largest since the 2008 Mumbai attack, and the biggest by jihadists since hundreds were killed in a Moscow theatre in 2002 and at a school in the Russian town of Beslan in 2004.

Algerian driver Iba El Haza said the hostage-takers spoke in different Arabic dialects.

"From their accents I understood one was Egyptian, one Tunisian, another Algerian and one was speaking English or (another) foreign language," he said after escaping on Thursday.

"The terrorists said: 'You have nothing to do with this, you are Algerians and Muslims. We won't keep you, we only want the foreigners'."

Hollande said French troops would stay in neighbouring Mali as long as was needed "to defeat terrorism" in the West African country and its neighbours.

Malian and French troops patrolled the outskirts of the contested northern town of Diabaly Sunday in a show of muscle a day after West African leaders demanded speedy UN aid to rout Islamists holding the vast desert north.