BAMAKO, Mali -- Kidnappers and at least some of their hostages were killed on Thursday as Algerian forces raided a gas facility where a heavily armed group of Islamist extremists was holding dozens of captives, including Americans and other foreigners, the Algerian government announced.
In a statement on national radio, the communications minister, Mohand Saïd Oublaïd, said that many of the hostages had been freed, but he warned that the military assault was not yet complete and that some captives remained trapped inside the remote facility in the Algerian desert.
"The operation resulted in the neutralization of a large number of terrorists and the liberation of a considerable number of hostages," Mr. Oublaïd said. "Unfortunately, we deplore also the death of some, as well as some who were wounded. We do not have final numbers."
He also said "the operation is ongoing, given the complexity of the site, to liberate the rest of the hostages and those who are trapped inside."
The announcement was the most detailed official information given by Algeria on the crisis. It began more than 24 hours earlier when Islamist militants seized the hostages at the internationally managed gas field in the Sahara near the Libyan border, in what they called retaliation for the French military intervention in neighboring Mali. The seizure of the gas field was one of the boldest abductions of foreign workers in recent years.
Unconfirmed news reports earlier on Thursday, quoting a statement reportedly from the hostage takers, said the Algerian military assault had left 35 hostages and 15 kidnappers dead. One Algerian government official called those numbers "exaggerated."
Algerian national radio described a scene of pandemonium and high alert at the public hospital in the town of In Amenas, near the gas-field facility, where wounded and escaped hostages were sent. The director of the hospital, Dr. Shahir Moneir, said in the radio report that wounded foreign hostages were transferred to the capital, Algiers.
In a brief telephone interview from the hospital, one of the Algerians who had been held captive at the facility, who identified himself as Mohamed Elias, said some of the hostages had exploited the chaos created by the Algerian military assault to flee. "We used the opportunity," he said, "and we just escaped."
The communications minister said the military assault force sent to end the siege had first sought a peaceful end.
"But confronted with the determination of the heavily armed terrorist group, our armed forces were forced to surround the site and fire warning shots," he said. "In front of the stubborn refusal of these terrorists to heed these warnings and confronted with their evident desire to leave Algeria with the foreign hostages to then use them as a bargaining chip, an assault was launched this Thursday at the end of the morning."
The minister's announcement came as foreign governments, including the United States, were seeking details on the fate of their citizens trapped inside the gas-field facility. There was no sign that the Algerians had given prior notice to any of the countries whose citizens were among the hostages.
Senior American military officials said Pentagon aides traveling in London with Defense Secretary Leon E. Panetta were struggling to get basic information about the Algerian raid, and that an unarmed American Predator drone was monitoring the gas-field site.
The senior official said that possibly seven to eight Americans were among the hostages -- the first official indication of the number of Americans held captive -- and that he did not know if any had been killed in the rescue operation.
Prime Minister David Cameron of Britain said his office had not been told ahead of time, an implicit criticism of the Algerian government. A spokesman said Mr. Cameron had learned of the raid through Britain's own intelligence sources. Mr. Cameron told reporters the situation was "very dangerous" as he and other British officials appeared to prepare for bad news.
"Although details have yet to become final I am afraid we should be under no illusion that there will be some bad and distressing news to follow from this terrorist attack," said Alistair Burt, a Foreign Office minister,
The gravity of the crisis prompted Mr. Cameron to cancel plans to fly to Amsterdam for a major speech on Friday about Britain's future in the European Union.
Japan also expressed strong concern, saying Algeria had not only failed to advise of the operation ahead of time, but had not heeded Japan's request to halt the operation because it was endangering the hostages.
"We asked Algeria to put human lives first and asked Algeria to strictly refrain," the chief Cabinet secretary, Yoshihide Suga, quoted Prime Minister Shinzo Abe as telling his Algerian counterpart, Abdelmalek Sellal, by telephone late Thursday.
The situation is "very confused," President François Hollande of France said at a news conference in Paris and was "evolving hour by hour." Mr. Hollande gave the first official confirmation that French citizens were among the captives.
Even before reports of the Algerian military's raid began to emerge, many hostages -- Algerian and foreign -- were reported to have escaped as the kidnappers failed to persuade Algerian authorities to give them safe passage with their captives.
The Algerian news Web site T.S.A. quoted a local official, Sidi Knaoui, as saying that 10 foreigners and 40 Algerians had escaped Thursday after the kidnappers had made several aborted efforts to flee with their captives.
The Irish government confirmed that an Irish citizen had escaped or been released. The man had contacted his family and was "understood to be safe and well and no longer a hostage," Ireland's Department of Foreign Affairs and Trade said in a statement.
Earlier, a French television station, France 24, quoted an unidentified hostage as saying the attackers "were heavily armed and forced several hostages to wear explosives belts. They threatened to blow up the gas field if Algerian forces attempted to enter the site," the station reported.
Algerian officials said at least two people, including a Briton, were killed in the kidnappers' initial assault on the gas facility, which began with an ambush on a bus trying to ferry workers to an airport. It was depicted by the attackers as reprisal for the French intervention in Mali and to punish Algeria for allowing French warplanes to use its airspace to reach targets in northern Mali. Algeria's official news agency said at least 20 fighters had carried out the attack and kidnapping.
The British Foreign Office confirmed that a British citizen had been killed in what Foreign Secretary William Hague called an "extremely dangerous situation."
On Wednesday, Mr. Panetta called the violence a terrorist act and said the United States was weighing a response. His statement suggested that the Obama administration could be drawn into a military entanglement in North Africa that it had been seeking to keep at arm's length -- even as it has acknowledged that the region has become a new haven for extremists who threaten Western security and vital interests.
The attack appeared to make good on a pledge by the Islamist militants who seized northern Mali last year to sharply expand their struggle against the West in response to the French military intervention that began last week.
In a statement sent to A.N.I., a Mauritanian news agency, a group called Al Mulathameen, which has links to Al Qaeda in the Islamic Maghreb, the North African affiliate of Al Qaeda, claimed it was holding more than 40 "crusaders" -- apparently a reference to non-Muslims -- "including seven Americans, two French, two British as well as other citizens of various European nationalities."
Algeria's interior minister, Daho Ould Kablia, said that the seizure of the gas field had been overseen by Mokhtar Belmokhtar, an Algerian who fought in Afghanistan against Soviet forces in the 1980s and has reportedly established his own group in the Sahara after falling out with other local Qaeda leaders.
Mr. Belmokhtar is known to French intelligence officials as "the uncatchable" and to some locals as "Mr. Marlboro" for his illicit cigarette-running business. His ties to Islamist extremists who seized towns across northern Mali last year are unclear, though he is thought to be based in the Malian city of Gao.
The facility at the center of the kidnapping is the fourth-largest gas development in Algeria, a major oil producer and OPEC member. The In Amenas gas compression plant is operated by BP of Britain, the Norwegian company Statoil and the Algerian national oil company Sonatrach.
Adam Nossiter reported from Bamako, and Alan Cowell and Scott Sayare from Paris. Reporting was contributed by Clifford Krauss from Houston, Rick Gladstone from New York, Elisabeth Bumiller and John F. Burns from London, Eric Schmitt from Washington, Steven Erlanger from Paris, Hiroko Tabuchi from Tokyo and Mayy El Sheikh from Cairo.
This article originally appeared in The New York Times.