Algerian special forces on Saturday found 15 burned bodies at a desert gas plant raided by Al Qaeda-linked fighters, two days after the army launched an assault to free hostages being held there by the Islamists, a source familiar with the crisis said.

Efforts were underway to identify the bodies, the source told Reuters. It was not clear how they had died.

More than 20 foreigners were still captive or missing inside the plant as a standoff between the army and the Islamist gunmen - one of the biggest international hostage crises in decades - entered its fourth day, having thrust Saharan militancy to the top of the global agenda.

The number and fate of those involved - hostages alive or dead as well as fighters - has yet to be confirmed, with the Algerian government keeping officials from Western countries far from the site where their countrymen were in peril.

Reports have put the number of hostages killed at between 12 to 30, with possibly dozens of foreigners still unaccounted for, among them Norwegians, Japanese, Britons, Americans and others.

The US State Department said on Friday one American, Frederick Buttaccio, had died but gave no further details. The French defence minister said he understood there were no more French workers among the hostages.

Two Norwegians were released overnight, leaving six unaccounted for, while Romania said three of its nationals had been freed. A number of Japanese engineering workers were still unaccounted for.

On Saturday, the Algerian military was holding the vast residential barracks at the In Amenas gas processing plant, while gunmen were holed up in the industrial plant itself with an undisclosed number of hostages, making it difficult for Algerian special forces to intervene.

The field commander of the Islamist group that attacked the plant is a veteran fighter from Niger called Abdul Rahman al-Nigeri, Mauritanian news agencies reported.

The army is surrounding the plant, and helicopters are monitoring the area, Reuters reporters near the scene said. A cordon appeared to have been thrown around the plant at a distance of about 10 km. Ambulances were on hand.

Scores of Westerners and hundreds of Algerian workers were inside the heavily fortified compound when it was seized before dawn on Wednesday by Islamist fighters who said they wanted a halt to a French military operation in neighbouring Mali.

Hundreds escaped on Thursday when the army launched its operation, but many hostages were killed in the assault. Algerian forces destroyed four trucks holding hostages, according to the family of a Northern Irish engineer who escaped from a fifth truck and survived.

The Northern Irishman, Stephen McFaul, told his family the attackers had strapped Semtex plastic explosive to his neck, bound his hands and taped his mouth.

Leaders of Britain, Japan and other countries have expressed frustration that the assault was ordered without consultation and officials have grumbled at the lack of information. Many countries also withheld details about their missing citizens to avoid releasing information that might aid the captors.

France's defence minister, Jean-Yves Le Drian, declined to criticise the Algerian response to the crisis, however.

"The Algerian authorities are on their own soil and responding in the fashion they can. The overriding mission is to tackle the terrorists," he told France 3 television.-Reuters