American officials also might be hesitant about demands, reported Friday by Mauritania’s ANI news agency, that the Islamists were seeking the release of two people convicted in U.S. federal court of terrorist attacks: Sheikh Omar Abdel-Rahman, the so-called “blind sheikh” who is serving a life term in connection with the 1993 bombing of the World Trade Center, and Aafia Siddiqui, a U.S.-trained Pakistani neuroscientist who was convicted of trying to kill her American interrogators in Afghanistan.

It remained unclear how many hostages and kidnappers died in the military assault and how many had escaped. It was also unknown how many hostages were still being held. Algeria said it was attempting to resolve the situation peacefully amid reports that the hostage-takers still held the industrial portion of the Ain Amenas complex.

According to Algerian state media, more than 500 people were in the compound, jointly operated by British oil giant BP, the Norwegian company Statoil and Algeria’s state oil company, when the kidnappers surrounded the site. At least 12 hostages are dead, the Algerian accounts said, though they did not give the nationalities of the victims.

Employees who escaped the compound said the surprise attack came early Wednesday morning from all sides as men surrounded them at their residential compound and where they work, quickly separating the hundreds of Algerians from the 132 foreign workers at the site, according to Algerian state media. According to the Associated Press, of those foreign workers, 100 are now free.

The Algerian counterattack came more than 24 hours later, at midday Thursday. One Irishman, Stephen McFaul, told his family that he’d seen four vehicles loaded with hostages blown up by attacking Algerian forces, according to The Irish Times newspaper.

One Algerian worker told state media that 260 of his fellow citizens were being held in a room when attacking Algerian troops blew off the door. The Algerians fled. Another said that they then waved a sign so that the Algerian military would know that they were employees and rescue them. “It happened so fast,” the worker was quoted as saying.

A French hostage told a European television station that he hid under his bed for 40 hours until Algerian soldiers rescued him.

A Briton riding a bus shortly after his rescue said he escaped during the military attack. Another British hostage, interviewed on local television, said he was “very impressed with the Algerian army.”

Cameron said Algerian Prime Minister Abdelmalek Sellal told him that the Algerian government launched the attack because it “judged there to be an immediate threat to the hostages and had felt obliged to respond.”

Meanwhile, Algeria reported that the United States had dispatched a plane to pick up survivors.

Algeria has identified the kidnappers as members of an al Qaida-affiliated group led by Mokhtar Belmokhtar, a one-eyed longtime militant who is described as charismatic, ruthless and stubborn. In a statement Wednesday, hours after the kidnappers snatched the gas line workers, Belmokhtar’s group said the attack was in retaliation for the French-led offensive in Mali.

Hannah Allam contributed to this report from Washington.