ALGIERS, Algeria (CBS) Algerian special forces launched a rescue operation Thursday at a natural gas plant in the Sahara Desert and freed foreign hostages held by al Qaeda-linked militants, but estimates for the number of dead varied wildly from four to dozens.
Militants claiming revenge for France's intervention against rebels in Mali seized the Ain Amenas natural gas complex on Wednesday, taking dozens of foreign workers hostage.
Algerian state television said Thursday that four captives, two Britons and two Filipinos, had died. But the militants said at least 35 hostages had died in the state's rescue attempt. There was no way to independently verify the toll in the remote location, 800 miles from Algiers.
The reports of high casualties have deeply disturbed foreign governments, prompting a number to criticize Algeria's operation. Britain's Foreign Office attempted to prepare the British public by saying, "We should be under no illusion that there will be some bad and distressing news to follow from this terrorist attack."
A diplomatic source confirmed to CBS News that the Algerian military had a plan to retake the facility and that there had been casualties among both the terrorists and hostages, including multiple deaths.
A British security source, citing a contact close to the scene, told CBS News "that the Algerians were firing from helicopters at anything that moved," but could not confirm any deaths.
Meanwhile, an unarmed American Predator drone arrived over the gas plant late Thursday afternoon, giving the U.S. its first independent look at the situation, CBS News national security correspondent David Martin reports.
Martin also reports that the Algerians said they didn't want any U.S. military assistance.
An administration official said it's unclear where the remaining hostages or the hostage-takers were and that the situation is still unfolding, CBS Radio News correspondent Cami McCormick reports.
Oil prices rose $1.08 on the news 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.
The Algerian government said it was forced to intervene due to the militants' stubbornness and their desire to escape with the hostages.
One Irish hostage was confirmed safe: supervising electrician Stephen McFaul, whose mother said he would not be returning to Algeria.
"He phoned me at 9 o'clock to say al Qaeda were holding him, kidnapped, and to contact the Irish government, for they wanted publicity. Nightmare, so it was. Never want to do it again. He'll not be back! He'll take a job here in Belfast like the rest of us," said his mother, Marie.
In Washington, White House Press Secretary Jay Carney said the Obama administration was "concerned about reports of loss of life and are seeking clarity from the government of Algeria."
Japan's Prime Minister Shinzo Abe protested the military raid as an act that "threatened the lives of the hostages," according to a spokesman.
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.
Norwegian Prime Minister Jens Stoltenberg would not criticize the Algerian operation but said he wished the Norwegian government would have been notified before it started. He added that while Algeria has declined medical help, Norway will send a plane with medical equipment and personnel.
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 south of the capital of Algiers.
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.
A spokesman for the Masked Brigade told the Nouakchott Information Agency in Mauritania that the seven surviving hostages included three Belgians, two Americans, a Briton and a Japanese citizen.
Algerian Interior Minister Daho Ould Kabila said the roughly 20 well-armed gunmen operating under orders from Moktar Belmoktar, al Qaeda's strongman in the Sahara, who is now based in Mali.
It is certainly the largest haul of hostages since 2003, when the radical group that later evolved into al Qaeda 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 and a Japanese company, JGC Corp, provides services for the facility.
Mali and al Qaeda specialist Mathieu 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."
© 2013 CBS Interactive Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.