Two days after Algerian forces ended a hostage crisis at a desert gas plant, the exact toll of those killed in the bloodbath remained unknown Monday, amid fears the total number of hostages who died could reach 50.

Prime Minister Abdelmalek Sellal was expected to unveil the final grim tally at a 1330 GMT press conference on Monday.

"I fear that it (the death toll) may be revised upward," Algerian Communications Minister Mohamed Said told a radio station earlier.

Governments scrambled to track down missing citizens as more details emerged after the final showdown on Saturday between special forces and extremists who had taken hundreds hostage, demanding an end to French military intervention in Mali.

The interior ministry reported Saturday that 23 foreign and Algerian hostages were killed during the siege of the In Amenas gas plant in the deep Sahara desert, which ended with Algerian forces storming the remaining part of the complex still in militant hands.

Algeria's private Ennahar television reported that 25 bodies were discovered at the plant on Sunday, while French daily El-Watan gave a toll of 30 hostages found dead at the site the same day.

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

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

Witnesses have said nine Japanese people connected to plant builder JGC were killed in the 72-hour ordeal. One Japanese man who survived gave a chilling account published Monday in the Daily Yomiuri newspaper.

The unidentified man was quoted as telling colleagues how the gunmen had dragged him from his barricaded room, handcuffed him and executed two hostages standing nearby.

"I was prepared to die," he said, before his captors abandoned him and other hostages who had been bundled into a vehicle that came under a hail of bullets. He then trudged for an hour through the desert to safety.

A Japanese delegation led by the deputy foreign minister arrived at the In Amenas hospital on Monday to arrange the repatriation of the bodies of the Japanese victims, a journalist reported.

The Philippine government said on Monday that six Filipino hostages were among the dead, killed "mostly by gunshot wounds and the effects of the explosions."

Ennahar reported on Monday that two of the slain hostage-takers were Canadian nationals. It also reported that five Islamists were captured alive at the plant on Sunday.

The Algerian interior ministry had said on Saturday that 32 kidnappers were killed in the standoff and the army freed 685 Algerian workers and 107 foreigners.

French Defence Minister Jean-Yves Le Drian described the hostage-taking as an "act of war" because of the large number of hostages involved -- the biggest since the 2008 attack by Islamist extremists on the Indian city of Mumbai.

The one-eyed 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.

Most hostages were freed on Thursday in the first Algerian rescue operation, which was initially viewed by foreign governments as hasty, before the focus of public condemnation turned on the jihadists.

The In Amenas plant is run by Britain's BP, Norway's Statoil and Sonatrach of Algeria.

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

One of the militants 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'."