Nearly 100 of the 132 foreign hostages and 573 Algerians were freed in an assault by government forces, the Los Angeles Times reported.
The militants took the hostages Wednesday and Algerian forces with aircraft attacked the complex the following day.
Alexandre Berce told Europe 1 radio he hid under the bed in his living quarters for 40 hours, opening his door only to Algerian co-workers who brought him food and water.
"I heard an enormous amount of fire but I had no idea what was happening," Berceaux said. "I was afraid. I could see myself ending up between four planks of wood."
It was unclear how many hostages were killed or exactly how many were still being held at the Tigantourine facility -- operated jointly by BP, the Algerian state oil company Sonatrach and Norway's Statoil.
Algerian Radio reported there was "ongoing activity at various locations" near the plant that some of the hostage-takers were using as a hideout.
A senior U.S. official questioned information coming from the Algerians, saying, "We hear one thing and then we hear something else."
U.S. Defense Secretary Leon Panetta said the United States was doing everything it could to ensure American hostages were returned home safely.
Panetta warned that terrorists "will find no refuge, not in Algeria, not in North Africa, not anywhere. Those who attack our country or our people will have no refuge."
A U.S. defense official said 10 to 20 people injured in the abduction were being evacuated to a U.S. facility in Europe. Their conditions were to be evaluated, the official said.
The Algerian Press Service reported two Britons and two Filipinos were killed Thursday. Two others, a Briton and an Algerian, died Wednesday when the militants ambushed a bus taking foreign workers at the facility to the local airport.
The Tigantourine complex is about 25 miles southwest of In Amenas and 800 miles southeast of Algiers.
Algerian Communications Minister Mohamed Said Oubelaid said the militants wanted to destabilize Algeria, "embroiling it in the Mali conflict and damaging its natural gas infrastructure."
A spokesman for the militants said Friday they would carry out further operations, warning Algerians to "stay away from the installations of foreign companies as we will strike where it is least expected."