Islamist militants seized at least 20 hostages, including as many as seven Americans, at an Algerian natural gas complex Wednesday in a brazen attack linked by the assailants to France’s military intervention in neighboring Mali.

At least one foreign worker was reported killed in the early morning assault on the vast In Amenas gas field near Algeria’s eastern border with Libya. The attack, attributed to a unit of al-Qaeda’s offshoot in North Africa, raised concerns about a broadening of Mali’s civil war five days after French forces joined the country’s fight against Islamist insurgents.

Militants claiming to belong to al-Qaeda in the Islamic Maghreb said they seized the hostages to punish Algeria for letting France use its airspace to launch strikes and support operations in Mali. The group said it would free the detainees only after France ceased all military involvement in Mali.

“Algeria’s participation in the war on the side of France betrays the blood of the Algerian martyrs who fell in the fight against the French occupation,” the Nouakchott News Agency quoted a militant spokesman as saying.

Algeria, which has sought to avoid entanglement in the Mali conflict, dispatched several hundred troops to the complex late Wednesday, sealing off the area but stopping short of a counter­attack.

Up in arms in northern Mali

Obama administration officials said they could not confirm the al-Qaeda group’s involvement in the hostage-taking, though they did not dispute the authenticity of a videotape from AQIM asserting responsibility.

Defense Secretary Leon E. Panetta denounced the attack during an official visit to Rome but said he did not know whether it bore “any relationship to the situation in Mali.”

“The United States strongly condemns these kinds of terrorist acts,” Panetta said. “I want to assure the American people that the United States will take all necessary and proper steps that are required to deal with this situation.”

‘A serious player’

If AQIM’s role in the attack is confirmed, it would signal the entry into the conflict of one of al-Qaeda’s most notorious commanders in North Africa — Mokhtar Belmokhtar, a one-eyed veteran of Afghanistan’s civil war and a terrorist leader regarded as one of the most violent and best-armed on the continent.

“He’s a serious player and a dangerous person who is very close to al-Qaeda historically,” said Bruce Hoffman, a Georgetown University terrorism expert.

Belmokhtar’s faction is thought to have commandeered vast quantities of weapons from Libyan military stockpiles after the collapse of Moammar Gaddafi’s government last year.

Early morning assault

Details of the attack on the gas complex remained murky, but Algerian news accounts said heavily armed assailants burst into the facility about 5 a.m. The attackers were in at least three vehicles and took dozens of hostages at a production facility and an administration building.

British energy giant BP, which runs the facility jointly with Norway’s Statoil and Algeria’s Sonatrach, said the hostage-
takers were still occupying the complex as evening fell.

Reports citing Algerian security sources said the assailants quickly released a large number of Algerian hostages while retaining 20 to 40 foreign nationals, including Americans, Europeans and Japanese.

The Algerian Interior Ministry said that at least one person was dead, but it did not specify the nationality.

Secretary of State Hillary Rodham Clinton had spoken about the situation to the U.S. ambassador in Algeria and the Algerian prime minister, said State Department spokeswoman Victoria Nuland.

“The best information we have at this time is that U.S. citizens are among the hostages,” Nuland said. “In order to protect their safety, I’m not going to get into numbers, I’m not going to get into names.”

‘It’s really concerning’

Al-Qaeda in the Islamic Maghreb is one of the terrorist network’s largest, best-funded and most heavily armed offshoots, amassing millions of dollars in ransom payments from earlier hostage-takings.

Counterterrorism experts described Belmokhtar as leader of AQIM’s southern branch and said he has deep ties to al-Qaeda’s main branch. Belmokhtar was born in Algeria in 1972 and traveled to Afghanistan as a teenager to join training camps run by extremists. In recent years, as al-Qaeda franchises in Africa have become more active, his profile has risen.

Belmokhtar has been involved in brokering the release of European and Canadian hostages, according to Andrew Black, an American businessman who has studied Belmokhtar and written a biographical article on him for the Jamestown Foundation.

Black said in an interview that the brazenness and apparent sophistication of Wednesday’s siege was stunning.

“To me, it’s really concerning,” Black said. “He hasn’t done something like this for years.”

Bruce Riedel, a former CIA officer and adviser to the Obama administration, said the attack could force Algeria to take a more aggressive role against a militant movement that has stepped up attacks on both sides of the Algeria-­Mali border in recent months.

“The Algerian government has been until now very, very cautious and reticent about moving against militant strongholds in Mali,” said Riedel, who is a scholar at the Brookings Institution. “That position is going to be hard to maintain now.

Edward Cody in Paris, Anthony Faiola in London and Craig Whitlock in Rome contributed to this report.