Yet information on the Algerian operation varied wildly and the conflicting reports that emerged from the remote area were impossible to verify independently.
Jean-Christophe Gray, a spokesman for British Prime Minister David Cameron, said Britain was not informed in advance of the raid but described the situation as "very grave and serious." French President Francois Hollande called it a "dramatic" situation involving dozens of hostages.
Islamists with the Masked Brigade, who have been speaking through a Mauritanian news outlet, said the Algerians opened fire Thursday as the militants tried to leave the vast Ain Amenas energy complex with their hostages. They claimed that 35 hostages and 15 militants died but seven hostages survived when Algerian helicopters strafed their convoy.
Algeria's official news service, meanwhile, claimed that 600 local workers were freed in the raid and half of the foreigners being held were rescued. Many of those locals were reportedly released on Wednesday, however, by the militants themselves.
One hostage was confirmed to be safe: Ireland said an Irish hostage at the plant was free and had made contact with his family.
An unarmed American surveillance drone soared overhead as the Algerian forces closed in, U.S. officials said. President Barack Obama's government offered military assistance Wednesday to help rescue the hostages - whose numbers varied wildly from dozens to hundreds - but the Algerian government refused, a U.S. official said in Washington. He spoke on condition of anonymity because he was not authorized to speak publicly about the offer.
News of the operation caused oil prices to rise $1.08 to $95.32 on the New York Mercantile Exchange and prompted energy companies like BP PLC and Spain's Compania Espanola de Petroleos SA to try to relocate energy workers at other Algerian plants.
Algerian forces who had ringed the Ain Amenas complex in a tense standoff had vowed not to negotiate with the kidnappers, who reportedly were seeking safe passage. Security experts said the end of the two-day standoff was in keeping with the North African country's tough approach to terrorism.
The kidnapping is one of the largest ever attempted by a militant group in North Africa. The militants phoned a Mauritanian news outlet to demand that France end its intervention in neighboring Mali to ensure the safety of the hostages in the isolated plant, located 800 miles (1,300 kilometers) south of the capital of Algiers.
Phone contacts with the militants were severed as government forces closed in, according to the Mauritanian news service, which often carries reports from al-Qaida-linked extremist groups in North Africa.
Both sides agreed only that the raid led to more bloodshed a day after the militants tried to hijack a busload of workers, were repulsed, then moved onto the sprawling desert plant and took hostages.
A 58-year-old Norwegian engineer who made it to the safety of a nearby Algerian military camp told his wife how militants attacked a bus Wednesday before being fended off by a military escort.
"Bullets were flying over their heads as they hid on the floor of the bus," Vigdis Sletten told The Associated Press in a phone interview from her home in Bokn, on Norway's west coast.
Her husband and the other bus passengers climbed out of a window and were transported to a nearby military camp, she said.
"He is among the lucky ones, and he has confirmed he is not injured," she said, declining to give his name for security reasons.
It was then that the militants went after the living quarters of the plant instead of disappearing back into the desert.
Information about the 41 foreign hostages the militants claimed to have - which allegedly included seven Americans - was scarce and conflicting. All were reportedly workers at the plant.
The spokesman for the Masked Brigade said 35 of the hostages died in the Algerian strafing. In a phone call with the sound of shelling behind him, he told the Nouakchott Information Agency that the seven surviving hostages on Thursday included three Belgians, two Americans, a Briton and a Japanese citizen.
Algeria's national news service, however, said only four hostages were freed during the military operation Thursday, citing a local law enforcement source.
Earlier in the day before the raid, an Algerian security official had said that 20 foreign hostages had escaped. He was not returning calls after the raid.
The Norwegian energy company Statoil had said 12 of its employees had been captured by the militants - nine Norwegians and three locals - while Japanese media reported at least 3 Japanese among the hostages and Malaysia confirmed two.
Algerian Interior Minister Daho Ould dismissed theories that the militants had come from Libya, 60 miles (100 kilometers) away, or from Mali, more than 600 miles (1,000 kilometers) away. He said the roughly 20 well-armed gunmen were from Algeria itself, operating under orders from Moktar Belmoktar, al-Qaida's strongman in the Sahara.
Yves Bonnet, the former head of France's spy service, also dismissed the idea that the operation was specifically linked to the French action in Mali due to the amount of organization it involved.
"It was an operation conceived well in advance - spectacular and needing a lot of preparation ... It was not at all an improvised operation," he told the Europe 1 radio. "The operation was probably already scheduled and simply getting all those people into the desert would take several days."
It is certainly the largest haul of hostages since 2003, when the radical group that later evolved into al-Qaida in North Africa snatched 32 Western tourists in southern Algeria. This is also the first time Americans have been involved.
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.
Mali and al-Qaida specialist Mathieu Guidere said Algeria's decisive response was in keeping with its usual response to terrorism.
"The message is 'We will terrorize the terrorists,'" he said, adding that the Algerian government had prioritized protecting its gas fields throughout the worst of a violent Islamist insurgency in the 1990s.
Guidere said Algeria's refusal to accept help was also normal.
"They never accept any military help," he said. "They want to do it their way."