A standoff between suspected Islamist militants holding several foreigners hostages and the Algerian army was continuing on Wednesday at a gas field operated by two major Western energy companies, fanning fears that violence could spread to neighboring countries amid France's efforts to uproot an al Qaeda-linked insurgency in Mali.
A group of militants attacked the In Amenas gas field in east-central Algeria, along the Libyan border, early Wednesday, according to Algeria's Interior Ministry and an energy worker briefed on developments by colleagues at the scene.
After a failed attack on a bus which was coming out of the base, the group entrenched inside living quarters with an undetermined number of Algerian and foreign workers, the ministry said.
An al Qaeda insurgency in Mali claimed responsibility for the attack. Omar Hamaha, a veteran kidnapper with al Qaeda in the Islamic Maghreb, said the organization's Saharan franchise carried out the raid and that seven Americans were among those seized. The attack was in retaliation for the French intervention in Mali, he said. His claims couldn't be immediately verified.
A State Department official said the U.S. "was still gathering more information and keeping a close watch on the situation." U.S. officials in Algeria are in contact with BP PLC, the British embassy and other local Algerian counterparts, the official said.
The official didn't comment on whether Americans were among those detained.
The army has surrounded the base and is working on solving the situation, the Algerian Interior Ministry said.
A U.K. government spokesman said that "a terrorist incident is ongoing" near In Amenas, which is operated by BP, Norway's Statoil ASA and the Algerian energy company Sonatrach.
British Prime Minister David Cameron headed a 45-minute meeting of the government's emergency contingency committee Wednesday afternoon to discuss the incident in Algeria. "The ongoing incident has involved various nationalities including several British nationals," said Mr. Cameron's spokesman. "We are working with BP to support the families of staff and providing consular assistance."
BP said that contact with the site was extremely difficult. "But we understand that armed individuals are still occupying the In Amenas operations site," said BP spokeswoman Sheila Williams said. Sonatrach didn't have any immediate comment.
Statoil head of international operations Lars Christian Bacher said two of its employees were injured. "This is one of the most serious situations that Statoil has ever been in," said Mr. Bacher.
There was confusion as to the exact number of casualties and hostages.
Algeria's state news service APS cited local government authorities in In Amenas saying two foreigners, including one British national, were killed during the attack and six injured.
Mr. Hamaha, the al Qaeda militant, said 41 hostages were held, including Japanese, British, French and U.S. nationals.
The Associated Press reported that at least eight foreigners, including Norwegian and Japanese nationals, were seized.
Irish Foreign Minister Eamon Gilmore issued a statement calling for the immediate release of an Irish citizen "who is reported to have been amongst the group of oil workers kidnapped in Algeria this morning."
A spokesman for Japan's foreign ministry said it was "aware of the report," and gathering information.
In Amenas harbors nearly 50 crude as well as gas fields, linked to the northern coast through several pipelines.
Last Friday, French military forces joined Malian troops battling Islamist rebels in the West African nation, amid Western fears that insurgent groups with links to al Qaeda could destabilize the region and gain the ability to strike overseas.
French officials have said they feared the campaign could lead jihadist movements to target French and European interests in retaliation.
Since the start of France's military intervention in Mali, Algeria has provided unexpected support to the French campaign.
Algeria, which has traditionally championed a noninterventionist policy and had voiced reservations about any foreign military role in Mali, allowed French combat jets to fly through its airspace and announced on Monday that it would close its southern border with Mali.
France's President Francois Hollande repeated on Tuesday that closing the border was an important step to prevent al Qaeda militants, many of whom are Algerian nationals, and have built up strongholds in Mali's north, from seeking refuge in Algeria's part of the vast Sahara.
Analysts said they were skeptical that Algeria, although the mightiest military force in the region, would succeed in fully tightening the border.
"It's possible to reinforce control on that border, but closing it is virtually impossible," said analyst Mohamed Chafik Mesbah, a retired officer of the Algerian army.
Geraldine Amiel, Selina Williams, Miho Inada, Kjetil Malkenes Hovland, Nicholas Winning and Katarina Gustafsson contributed to this article.