Algeria's state news agency also reported late Friday that a "provisional toll" shows 12 hostages have been killed since the start of the Algerian military operation to free workers kidnapped by militants at the plant. The APS news agency quoted an unidentified security source for the new death toll and said the fatalities include both Algerian and foreign workers.
That hostage death toll would be more than double the one APS had reported earlier. The news agency has said 18 militants had been killed.
Yet the report that nearly 100 workers were safe could indicate a breakthrough in the confrontation that began when the militants seized the plant early Wednesday.
U.S. officials said Friday that one American worker at the complex, Texas resident Frederick Buttaccio, has been found dead there, but that it was unclear how he died. The Obama administration was seeking to secure the release of other Americans still being held by the militants.
France said one Frenchman has been killed during the raid by Algerian forces. The Foreign Ministry identified him as Yann Desjeux, without providing additional information. The ministry said three other French hostages are now free, but didn't say if any French citizens remain in captivity.
The militants, meanwhile, offered to trade two captive American workers for two terror figures jailed in the United States, according to a statement received by a Mauritanian news site that often reports news from North African extremists.
The U.S. State Department confirmed that some Americans are still being held hostage in Algeria. Asked about a militant offer to trade two American hostages for jailed terror figures in the United States, State Department spokeswoman Victoria Nuland said, "The United States does not negotiate with terrorists."
U.S. Secretary of State Hillary Rodham Clinton urged Algeria to do everything possible to protect the remaining hostages. Clinton said that in her conversation on Friday with Algerian Prime Minister Abdelmalek Sellal she emphasized that "the utmost care must be taken to preserve innocent life."
It was not clear whether the remaining foreigners were still captive or had died during the Algerian military offensive to free them that began Thursday.
The desert siege erupted Wednesday when the militants attempted to hijack two buses at the plant, were repulsed, and then seized the sprawling refinery, which is 800 miles south of Algiers. They had claimed the attack came in retaliation for France's recent military intervention against Islamist rebels in neighboring Mali, but security experts have said it must have taken weeks of planning to hit the remote site.
Since then, Algeria's government has kept a tight grip on information about the siege.
The militants had seized hundreds of workers from 10 nations at Algeria's remote Ain Amenas natural gas plant. The overwhelming majority were Algerian and were freed almost immediately.
Algerian forces retaliated Thursday by storming the plant in an attempted rescue operation that left leaders around the world expressing strong concerns about the hostages' safety.
Militants claimed 35 hostages died on Thursday when Algerian military helicopters opened fire as the Islamists transported the hostages around the gas plant. They militants said they continue to hold seven foreign hostages, according to Mauritanian news agency ANI, which said it had spoken to militants on Friday.
On Friday, trapped in the main refinery area, the militants offered to trade two American hostages for two prominent terror figures jailed in the United States. Those the militants sought included Omar Abdel Rahman, the blind sheikh who was convicted of plotting to blow up New York City landmarks and considered the spiritual leader of the 1993 World Trade Center bombing, and Aafia Siddiqui, a Pakistani scientist convicted of shooting at two U.S. soldiers in Afghanistan.
But U.S. Defense Secretary Leon Panetta said there would be "no place to hide" for anyone who looks to attack the United States. "Terrorists should be on notice that they will find no sanctuary, no refuge, not in Algeria, not in North Africa, not anywhere," Panetta said Friday.
Workers kidnapped by the militants came from around the world — Americans, Britons, French, Norwegians, Romanians, Malaysians, Japanese, Algerians.
World leaders have expressed strong concerns in the past few days about how Algeria was handing the situation and its apparent reluctance to communicate.
Terrorized hostages from Ireland and Norway trickled out of the Ain Amenas plant. BP, which jointly operates the plant, said it had begun to evacuate employees from Algeria.
"This is a large and complex site and they are still pursuing terrorists and possibly some of the hostages," British Prime Minister David Cameron said Friday in London.
He told British lawmakers the situation remained fluid and dangerous, saying "part of the threat has been eliminated in one part of the site, a threat still remains in another part."
A U.S. military C-130 transport plane flew a number of people including former Ain Amenas hostages from the Algerian capital of Algiers to a U.S. facility in Europe, a U.S. official said. He declined to be specific about the destination, their nationalities or the extent of the wounds that he said some had.
A flood of foreign energy workers were being evacuated from the North African nation amid security concerns.
BP evacuated one U.S. citizen along with other foreign energy workers from Algeria to Mallorca and then London. The oil giant said three flights left Algeria on Thursday, carrying 11 BP employees and several hundred energy workers from other companies.
A fourth plane was taking more people out of the country on Friday, BP said.