PARIS -- Islamist militants seized a foreign-operated gas field in Algeria early Wednesday and took 20 or more foreign hostages, including Americans, according to an Algerian government official and the country's state-run news agency, in what appeared to be a retaliation for the French-led military intervention in neighboring Mali.
The Algerian agency said at least two people had been killed in the gas-field seizure, including one British national, and that the hostages included American, British, French, Norwegian and Japanese citizens.
State Department spokeswoman Victoria Nuland told reporters in Washington that an unidentified number of U.S. citizens were believed to be among the hostages, and Defense Secretary Leon E. Panetta, traveling in Italy, seemed to raise the possibility that the United States might take military action in response.
"By all indications, this is a terrorist act," Mr. Panetta said. "It is a very serious matter when Americans are taken hostage along with others." He also 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."
The exact number of people being held was still far from certain. A top Algerian government official said 20 Islamist militants had attacked the gas field, and that security services had now "encircled the base" so "no one can leave." Concerning the number of hostages, he said "the situation is confused for the moment," and that there might be as many as 30. "We don't have precise figures for now."
As for the 20 attackers, he said, they came heavily armed, in three unmarked vehicles. "That's how they slipped through," he said.
All told, as many as 40 workers could be held hostage, according to oil company officials with interest in the field. A Japanese official confirmed that Japanese nationals were involved, and the Irish Foreign Ministry said one Irish citizen had been kidnapped. Some news agencies said as many as 41 hostages were seized.
The attack on the gas field appeared to be the first retribution by the Islamists for the French armed intervention in Mali last week, potentially broadening the conflict beyond Mali's borders and raising the possibility of drawing an increasing number of foreign countries directly into the conflict. Western officials had long warned that a Mali intervention, designed to halt an Islamist militant advance in that country, could incite a backlash far beyond Mali's borders.
Algeria, which has its own long history of fighting Islamic militancy, suggested that it would show no tolerance for the gas-field attackers. "The Algerian authorities will not respond to the demands of the terrorists and will not negotiate," Algeria's interior minister, Daho Ould Kablia, was quoted as saying by the official news agency.
The attack occurred at the fourth-largest gas development in Algeria, In Amenas, and at its gas compression plant, which is operated by BP, the Norwegian company Statoil and the Algerian national oil company Sonatrach.
Statoil spokesman Bard Glad Pedersen said that of 17 Statoil employees working in the field, only four were able to safely escape to a nearby Algerian military camp. "There is a hostage situation," he said. "We do not provide further information how we are dealing with the situation. Our main priority is the safety of our colleagues."
The Sahara Media Agency of Mauritania, quoting what it described as a spokesman for the militants, said they were holding five hostages in a production facility on the site and 36 others in a housing area, and that as many as 400 Algerian soldiers were surrounding the operation. But that information could not be confirmed, and the agency's report on the specifics of where the hostages were held raised questions about its credibility.
Fighters with links to al-Qaida's African affiliate, al-Qaida in the Islamic Maghreb, claimed responsibility for the attack, according to both Mauritanian and Algerian news agencies. They quoted militants claiming that the kidnappings were a response to the Algerian government's decision to allow France to use its airspace to conduct strikes against Islamists in Mali.
Islamist groups and bandits have long operated in the deserts of western Africa, and a collection of Islamists have occupied the vast expanse of northern Mali since last year. In retaliation for the French-led effort to drive them out, those groups, including al-Qaida in the Islamic Maghreb, have pledged to strike against France's interests on the continent and abroad, as well as those of nations backing the French operations. In France, security has been reinforced at airports, train stations and other public spaces.
The militant groups are financed in large part through ransoms paid for the freeing of Western hostages, and regular kidnappings have occurred in the West African desert in recent years. Seven French nationals are currently being held there.
The attack Wednesday was carried out by a "heavily armed" group of "terrorists" traveling aboard three vehicles, the Algeria Interior Ministry statement said, and targeted a bus transporting foreign workers to a nearby airport at 5 a.m.
Algeria, which shares a desert border of several hundred miles with Mali, has resisted the possibility of organizing an armed intervention into the Malian north, fearing that fighting could spill into Algeria or drive militants into the country. Indeed, al-Qaida in the Islamic Maghreb, one of the militant groups now holding northern Mali, began as an insurgent group fighting the Algerian government in the 1990s. But Algeria has authorized French jets flying missions in Mali to cross Algerian airspace.