WASHINGTON (AP) — The Obama administration was in the dark Thursday about the hostage situation at a natural gas plant in Algeria, where Algerian forces launched a military assault to free dozens of foreign hostages, including an unknown number of Americans, held by militants who have ties to Mali's rebel Islamists.

The U.S. condemned the militants for holding hostage several Americans and dozens of other people at the isolated plant, located 800 miles south of the capital, Algiers, in the Sahara Desert, but had been unable by midday to offer any details about how many American hostages they were, whether they were still in captivity or even alive.

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.

"This is a serious and sensitive situation," Pentagon spokesman George Little told reporters traveling with Defense Secretary Leon Panetta in England. Little said military officials were actively seeking information, and that Panetta had been briefed by senior military officials.

Militants earlier said they were holding seven Americans, but the administration confirmed only that Americans were among those taken.

White House press secretary Jay Carney said Thursday the administration was monitoring the situation closely and remained in contact with the Algerian government.

"I just can only say that we are deeply concerned about any loss of innocent life and are seeking clarity from the government of Algeria," Carney told reporters.

Secretary of State Hillary Rodham Clinton was expected to speak by telephone Thursday with Algerian Prime Minister Abdelmalek Sellal.

During their conversation Wednesday, Clinton expressed Washington's "willingness to be helpful," and the pair discussed what type of assistance might be needed, State Department spokeswoman Victoria Nuland said. She declined to elaborate any further.

A senior U.S. military official said the military offered Algeria hostage-rescue teams on Wednesday, but the offer was refused. The official spoke on condition of anonymity because he wasn't authorized to discuss the offer publicly.

"This is a hostage situation," Nuland told reporters. "Our first priority is for the safety and security of the people involved."

She said the United States also was in contact with American businesses across North Africa and the Middle East to help them guard against the possibility of copycat attacks.

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(Photo: AP Photo/Kjetil Alsvik, Statoil via NTB scanpix)