ALGIERS, Algeria (AP) — In a desert standoff deep in the Sahara, the Algerian army ringed a natural gas complex where Islamist militants hunkered down with dozens of hostages Wednesday night after a rare attack that appeared to be the first violent shock wave from the French intervention in Mali.

A militant group that claimed responsibility said 41 foreigners, including seven Americans, were being held after the assault on one of oil-rich Algeria’s energy facilities, 800 miles from the capital of Algiers and 1,000 miles (1,600 kilometers) from the coast. Two foreigners were killed.

The group claiming responsibility said the attack was in revenge for Algeria’s support of France’s military operation against al-Qaida-linked rebels in neighboring Mali. The U.S. defense secretary called it a ‘‘terrorist act.’’

The militants appeared to have no escape, with troops surrounding the complex and army helicopters clattering overhead.

The group — called Katibat Moulathamine or the Masked Brigade — phoned a Mauritanian news outlet to say one of its affiliates had carried out the operation at the Ain Amenas gas field, and that France should end its intervention in Mali to ensure the safety of the hostages.

BP, the Norwegian company Statoil and the Algerian state oil company Sonatrach, operate the gas field. A Japanese company, JGC Corp, provides services for the facility as well.

In Rome, U.S. Defense Secretary Leon Panetta declared that the U.S. ‘‘will take all necessary and proper steps’’ to deal with the attack in Algeria. He would not detail what such steps might be but condemned the action as ‘‘terrorist attack’’ and likened it to al-Qaida activities in Pakistan, Afghanistan and in the United States on Sept. 11, 2001.

Algeria’s top security official, Interior Minister Daho Ould Kabila, said that ‘‘security forces have surrounded the area and cornered the terrorists, who are in one wing of the complex’s living quarters.’’

He said one Briton and one Algerian were killed in the attack, while a Norwegian and two other Britons were among the six wounded.

‘‘We reject all negotiations with the group, which is holding some 20 hostages from several nationalities,’’ Kabila said on national television, raising the specter of a possible armed assault to try to free the hostages.

The head of a catering company working on the base told the French Journal de Dimanche that helicopters were flying over the complex and the army waited outside. There were even reports of clashes between the two sides and a member of the militant group told the Mauritanian news outlet the Islamists had already repelled one assault by Algerian soldiers late Wednesday night.

It was not immediately possible to account for the discrepancies in the number of reported hostages. Their identities also were not clear, but Ireland announced that they included a 36-year-old married Irish man. Japan, Britain and the U.S. said their citizens were taken. A Norwegian woman said her husband called her saying that he had been taken hostage.

Hundreds of Algerians work at the plant and were also captured in the attack, but the Algerian state news agency reported they were gradually released unharmed Wednesday.

The Algerian minister said it seemed the militants were hoping to negotiate their departure from the area — a notion he rejected. He also dismissed theories that the militants had come from Libya, a mere 60 miles (100 kilometers) away, or from Mali, more than 600 miles (1,000 kilometers) away.

Kabila said the roughly 20 well armed gunmen were from Algeria itself, operating under orders from Moktar Belmoktar, al-Qaida’s strongman in the Sahara.

In Washington, U.S. State Department spokeswoman Victoria Nuland confirmed that ‘‘U.S. citizens were among the hostages.’’

The caller to the Nouakchott Information Agency, which often carries announcements from extremist groups, said the kidnapping was carried out by ‘‘Those Who Signed in Blood,’’ a group created to attack countries participating in the offensive against Islamist groups in Mali.

The Masked Brigade was formed by Belmoktar, a one-eyed Algerian who recently declared he was leaving the terror network’s Algerian branch, al-Qaida in the Islamic Maghreb, to create his own group. He said at the time he would still maintain ties with the central organization based in Afghanistan and Pakistan.