ALGIERS/IN AMENAS, Algeria: More than 20 foreigners were still either being held hostage or missing inside a gas plant on Friday after Algerian forces stormed the desert complex to free hundreds of captives taken by Islamist militants.

More than a day after the Algerian army launched an assault to seize the remote desert compound, much was still unclear about the number and fate of the victims, leaving countries with citizens in harm's way struggling to find hard information.

Reports on the number of hostages killed ranged from 12 to 30, with anywhere from dozens to scores of foreigners still unaccounted for.

Norway's Prime Minister Jens Stoltenberg, eight of whose countrymen were missing, said fighters still controlled the gas treatment plant itself, while Algerian forces now held the nearby residential compound that housed hundred of workers.

Leaders of Britain, Japan and other countries expressed frustration that the assault had been ordered without consultation. Many countries were also withholding information about their citizens to avoid helping the captors.

Night fell quietly on the village of In Amenas, the nearest settlement, some 50 km (30 miles) from the vast and remote desert plant. A military helicopter could be seen in the sky.

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.

Norway's Stoltenberg said some of those killed in vehicles blasted by the army could not be identified. "We must be prepared for bad news this weekend but we still have hope."

Northern Irish engineer Stephen McFaul, who survived, said he saw four trucks full of hostages blown up by Algerian troops.

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.

A local Algerian source said 100 of 132 foreign hostages had been freed from the facility. However, other estimates of the number of unaccounted-for foreigners were higher. Earlier the same source said 60 were still missing. Some may be held hostage; others may still be hiding in the sprawling compound.

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.

Those still unaccounted for on Friday included 10 from Japan and eight Norwegians, according to their employers, and a number of Britons which Cameron put at "significantly" less than 30

France said it had no information on two Frenchmen who may have been at the site and Washington has said a number of Americans were among the hostages, without giving details. The local source said a U.S. aircraft landed nearby on Friday.

The attackers had initially claimed to be holding 41 Western hostages. Some Westerners were able to evade capture by hiding.

They lived among hundreds of Algerian employees on the compound. The state news agency said the army had rescued 650 hostages in total, 573 of whom were Algerians.

"(The army) is still trying to achieve a 'peaceful outcome' before neutralising the terrorist group that is holed up in the (facility) and freeing a group of hostages that is still being held," it said, quoting a security source.

Algerian commanders said they moved in on Thursday about 30 hours after the siege began, because the gunmen had demanded to be allowed to take their captives abroad.

A French hostage employed by a French catering company said he had hidden in his room for 40 hours under the bed, relying on Algerian employees to smuggle him food with a password.

"I put boards up pretty much all round," Alexandre Berceaux told Europe 1 radio. "I didn't know how long I was going to stay there ... I was afraid. I could see myself already ending up in a pine box."

The captors said their attack was a response to a French military offensive in neighbouring Mali. However, some U.S. and European officials say the elaborate raid probably required too much planning to have been organised from scratch in the single week since France first launched its strikes.

Paris says the incident proves that its decision to fight Islamists in neighbouring Mali was necessary.

Security in the half-dozen countries around the Sahara desert has long been a pre-occupation of the West. Smugglers and militants have earned millions in ransom from kidnappings.

The most powerful Islamist groups in the Sahara were severely weakened by Algeria's secularist military in a civil war in the 1990s. But in the past two years the regional wing of Al Qaeda gained fighters and arms as a result of the civil war in Libya, when arsenals were looted from Muammar Gaddafi's army.

Al Qaeda-linked fighters, many with roots in Algeria and Libya, took control of northern Mali last year, prompting the French intervention in that poor African former colony.

The Algerian security source said only two of 11 militants whose bodies were found on Thursday were Algerian, including the squad's leader. The others comprised three Egyptians, two Tunisians, two Libyans, a Malian and a Frenchman, he said.

The plant was heavily fortified, with security, controlled access and an army camp with hundreds of armed personnel between the accommodation and processing plant, Andy Coward Honeywell, who worked there in 2009, told the BBC.

The apparent ease with which the fighters swooped in from the dunes to take control of an important energy facility, which produces some 10 percent of the natural gas on which Algeria depends for its export income, has raised questions over the value of outwardly tough security measures.

Algerian officials said the attackers may have had inside help from among the hundreds of Algerians employed at the site.

The attackers benefitted from bases and staging grounds across the nearby border in Libya's desert, Algerian officials said.

U.S. Defense Secretary Leon Panetta said those respsonsible would be hunted down: "Terrorists should be on notice that they will find no sanctuary, no refuge, not in Algeria, not in North Africa, not anywhere.... Those who would wantonly attack our country and our people will have no place to hide."

The kidnappers threatened more attacks and warned Algerians to stay away from foreign companies' installations, according to Mauritania's news agency ANI, which maintained contact with the group during the siege.

Hundreds of workers from international oil companies were evacuated from Algeria on Thursday and many more will follow, said BP, which jointly ran the gas plant with Norway's Statoil and the Algerian state oil firm.

The overall commander of the kidnappers, Algerian officials said, was Mokhtar Belmokhtar, a one-eyed veteran of Afghanistan in the 1980s and Algeria's bloody civil war of the 1990s. He appears not to have been present.

Algerian security specialist Anis Rahmani, author of several books on terrorism and editor of Ennahar daily, told Reuters about 70 militants were involved from two groups, Belmokhtar's "Those who sign in blood", who travelled from Libya, and the lesser known "Movement of the Islamic Youth in the South".

Britain's Cameron, who warned people to prepare for bad news and who cancelled a major policy speech on Friday to deal with the situation, said he would have liked Algeria to have consulted before the raid. Japan made similar complaints.

U.S. officials had no clear information on the fate of Americans. Washington, like its European allies, has endorsed France's military intervention in Mali.