More than 20 foreigners were captive or missing inside a desert gas plant on Saturday, nearly two days after the Algerian army launched an assault to free them that saw many hostages killed.

The standoff between the Algerian army and al Qaeda-linked 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 victims 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 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.

By nightfall on Friday, 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.

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 an 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.

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.

An Algerian security source said 30 hostages, including at least seven Westerners, had been killed during Thursday's assault, along with at least 18 of their captors. Eight of the dead hostages were Algerian, with the nationalities of the rest of the dead still unclear, he said.

Algeria's state news agency APS put the total number of dead hostages at 12, including both foreigners and locals.

The base was home to foreign workers from Britain's BP, Norway's Statoil and Japanese engineering firm JGC Corp and others.

Norway says eight Norwegians are still missing. JGC said it was missing 10 staff. Britain and the United States have said they have citizens unaccounted for but have not said how many.

The Algerian security source said 100 foreigners had been freed but 32 were still unaccounted for.

"We must be prepared for bad news this weekend but we still have hope," Norwegian Prime Minister Jens Stoltenberg said.

The attack has plunged international capitals into crisis mode and is a serious escalation of unrest in northwestern Africa, where French forces have been in Mali since last week fighting an Islamist takeover of Timbuktu and other towns.

"We are still dealing with a fluid and dangerous situation where a part of the terrorist threat has been eliminated in one part of the site, but there still remains a threat in another part," British Prime Minister David Cameron told his parliament.

Two Japanese, two Britons and a French national were among the seven foreigners confirmed dead in the army's storming, the Algerian security source told Reuters. One British citizen was killed when the gunmen seized the hostages on Wednesday.

"(The army) is still trying to achieve a ‘peaceful outcome' before neutralizing the terrorist group that is holed up in the (facility) and freeing a group of hostages that is still being held," Algeria's state news agency said on Friday, quoting a security source.

Sixteen foreign hostages were freed on Saturday, a source close to the crisis said.

Those freed included two Americans, two Germans and one Portuguese, the source told Reuters. The nationalities of the others were not immediately clear.

Meanwhile, the Algerian army is clearing mines planted by al Qaeda-linked fighters at the desert gas plant they attacked three days ago, the national oil and gas company Sonatrach said on Saturday.

News of the mine clearing operation came shortly after Algerian official media said special forces had launched a final assault against the militants.