Thirty-seven foreigners of eight different nationalities, as well as an Algerian, were killed by hostage-takers in a well-planned attack on a remote gas plant, some of them brutally executed.

Algerian Prime Minister Abdelmalek Sellal said five other foreigners were still missing, and some of the hostages had been executed "with a bullet to the head" as the four-day crisis at the In Amenas gas plant deep in the Sahara ended in a bloodbath on Saturday.

The premier gave the final grim figures after Algeria had warned other nations to prepare for a higher body count, amid fears as many as 50 captives may have died in the world's deadliest hostage crisis in almost a decade.

"Thirty-seven foreigners of eight different nationalities" were killed during the siege, Sellal told reporters, with the death of an Algerian bringing the overall toll to 38.

He did not specify the nationalities of the foreigners, seven of whom remain unidentified, but other official sources have already confirmed seven Japanese, six Filipinos, three Britons, two Romanians and one Frenchman died.

The United States also confirmed on Monday three of its citizens were among the dead.

Survivors' photos seen by AFP showed bodies riddled with bullets, some with their heads half blown away by the impact of the gunfire.

A total of 29 militants were killed and three captured in the siege which ended in a final showdown between special forces and the remaining militants holed up in the sprawling gas complex.

Some foreign leaders initially accused Algeria of keeping them in the dark about an operation that many observers found hasty, but criticism then focused on the Islamist militants behind the hostage crisis.

Sellal defended the army's ruthless response to the attack, which he said had been planned over the past two months, calling the demands of the gunmen for the release of Islamist prisoners "unacceptable".

"Initially the security forces... tried negotiating in the hope of appeasing the hostage-takers. But these terrorists were determined. Their demands... were unreasonable and unacceptable," he said.

Eleven of the hostage-takers, who were also demanding an end to French military intervention in Mali, were Tunisian and another three Algerian, with the rest Canadian, Egyptian, Malian, Nigerien and Mauritanian.

Most had entered the country from neighbouring Mali, Sellal said, adding the group's leader was Mohamed el-Amine Bencheneb, an Algerian militant known to the country's security services, who was killed in the siege.

-- 'We're looking for Christians' --

Governments have been scrambling to track down missing citizens as more as more harrowing details emerged of the siege.

One Japanese survivor was quoted in the Daily Yomiuri newspaper as telling colleagues how the gunmen had dragged him from his barricaded room, handcuffed him and executed two hostages standing nearby.

"They were brutally executed," said an Algerian who identified himself as Brahim, after escaping the ordeal, referring to Japanese people gunned down by the hostage-takers.

A Filipino survivor described how the militants used foreigners as human shields, and said he was the only survivor out of nine hostages in a van that blew up on Thursday, apparently from C-4 explosives rigged to the vehicle.

"I was the only one who survived because I was sandwiched between two spare tyres," said the visibly distressed father of four.

The alleged mastermind of the hostage-taking, Mokhtar Belmokhtar, said in a video posted online that it was carried out by 40 fighters from the Muslim world and "European countries".

His Al-Qaeda-linked group "Signatories in Blood" threatened to stage attacks on nations involved in the French-led operation to evict Islamists from Algeria's neighbour Mali, and said it had been open to negotiations.

"But the Algerian army did not respond... preferring to stage an attack which led to the elimination of the hostages," it said in a message published by the Mauritanian news agency ANI.

The special forces managed to free 685 Algerian and 107 foreign hostages, most of them on Thursday, in the first Algerian rescue operation.

An Algerian employee of BP who identified himself as Abdelkader said he was at a security post with colleagues on Wednesday morning when he saw a jeep with seven people inside smash through the barrier and screech to a halt.

One of the militants got out of the vehicle, demanded their mobile phones and ordered them not to move, before disabling the security cameras.

"He said: 'You are Algerians and Muslims, you have nothing to fear. We're looking for Christians, who kill our brothers in Mali and Afghanistan and plunder our resources'."