With Western leaders clamouring for details of the assault they said Algeria had launched on Thursday without consulting them, a local source said the gas base was still surrounded by Algerian special forces and some hostages remained inside.

Thirty hostages, including several Westerners, were killed during the storming on Thursday, the source said, along with at least 11 of their captors, who said they had taken the site as retaliation for French intervention against Islamists in neighbouring Mali.

The crisis represents a serious escalation of unrest in North Africa, where French forces have been in Mali since last week to fight an Islamist takeover of the north, and strikes a heavy blow to Algeria’s vital oil industry, just recovering from years of civil war.

Fourteen Japanese were among those still unaccounted for by the early hours of Friday, their Japanese employer said, while Norwegian energy company Statoil, which runs the Tigantourine gas field with Britain’s BP and Algeria’s national oil company, said eight Norwegian employees were still missing.

A French hostage employed by a French catering company said Algerian military forces were combing the sprawling In Amenas site for hostages when he was escorted away by the military.

“They are still counting them up,” Alexandre Berceaux told Europe 1 radio.

The crisis posed a serious dilemma for former colonial power Paris and its allies as French troops attacked the hostage-takers’ al Qaeda allies in Mali, another former colony.

The kidnappers warned Algerians to stay away from foreign companies’ installations in the OPEC-member oil and gas producing state, threatening more attacks, Mauritania’s news agency ANI said, citing a spokesman for the group.

Algerian workers form the backbone of an oil and gas industry that has attracted international firms in recent years partly because of military-style security. The kidnapping, storming and further threat cast a deep shadow over its future.

An Irish engineer who survived said he saw four jeeps full of hostages blown up by Algerian troops whose commanders said they moved in about 30 hours after the siege began because the gunmen had demanded to be allowed to take their captives abroad.

The crisis posed a serious dilemma for Paris and its allies as French troops attacked the hostage-takers’ al-Qaeda allies in Mali. It also left question marks over the ability of OPEC-member Algeria to protect vital energy resources and strained its relations with Western powers.

Two Japanese, two Britons and a French national were among at least seven foreigners killed, the source told reporters. Eight dead hostages were Algerian. The nationalities of the rest, as well as of perhaps dozens more who escaped, were unclear. Some 600 local Algerian workers, less well guarded, survived.

Norwegian Foreign Minister Espen Barth Eide said his country still did not know the fate of eight of 13 Norwegian hostages taken. “As we understand it, the operation is still ongoing,” he told Britain’s BBC broadcaster.

Japan’s Prime Minister Shinzo Abe has cancelled part of his trip in Southeast Asia, his first overseas trip since taking office, and will fly home early due to the hostage crisis, Japan’s senior government spokesman said on Friday.

“The action of Algerian forces was regrettable,” said Japan’s Chief Cabinet Secretary Yoshihide Suga, adding Tokyo had not been informed of the operation in advance.

Americans, Romanians and an Austrian have also been mentioned by their governments as having been captured by the militants, who call themselves the “Battalion of Blood” and have demanded France end its week-old offensive in Mali.

Underlining the view of African and Western leaders that they face a multinational Islamist insurgency across the Sahara – a conflict that prompted France to send hundreds of troops to Mali last week – the official source said only two of the 11 dead militants were Algerian, including the squad’s leader.

The bodies of three Egyptians, two Tunisians, two Libyans, a Malian and a Frenchman – all assumed to have been hostage-takers – were found, the security source said.

The group had claimed to have dozens of guerrillas on site and it was unclear whether any militants had managed to escape.

The overall commander, Algerian officials said, was Mokhtar Belmokhtar, a veteran of Afghanistan in the 1980s and Algeria’s bloody civil war of the 1990s. He appears not to have been present and has now risen in stature among a host of Saharan Islamists, flush with arms and fighters from chaotic Libya, whom Western powers fear could spread violence far beyond the desert.

Algeria’s government made clear it remains implacably at odds with Islamist guerrillas who remain at large in the south, years after the civil war in which some 200,000 people died. Communication Minister Mohamed Said repeated their refusal ever to negotiate with hostage-takers.

“We say that in the face of terrorism, yesterday as today as tomorrow, there will be no negotiation, no blackmail, no respite in the struggle against terrorism,” he told APS news agency.

British Prime Minister David Cameron, who warned people to prepare for bad news and who cancelled a major policy speech on Friday to deal with the situation, said through a spokesman that he would have liked Algeria to have consulted before the raid.

A Briton and an Algerian were also killed on Wednesday.

US officials had no clear information on the fate of Americans, though a US military drone had flown over the area. Washington, like its European allies, has endorsed France’s move to protect the Malian capital by mounting air strikes last week and now sending 1,400 ground troops to attack Islamist rebels.

A US official said on Thursday it would provide transport aircraft to help France with a mission whose vital importance, President Francois Hollande said, was demonstrated by the attack in Algeria. Some fear, however, that going on the offensive in the remote region could provoke more bloodshed closer to home.

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