A search-and-rescue mission continued into Thursday morning after a KC-130 refueling plane crashed into a mountain in western Pakistan late Wednesday, killing all seven U.S. Marines aboard

A Central Command spokesman, Maj. Ralph Mills, said Marines and Pakistanis had approached the crash site, but no bodies had been recovered as of Wednesday night.

"We made it to the crash site on foot," Mills said. "But they were unable to remain there. It is a very steep grade, and they were unable to get footing. The site is secure."

It was the worst U.S. casualty toll from the war in Afghanistan and the casualties included the first U.S. servicewoman to die since the war began.

The Pentagon identified the seven Marines who were killed as: the pilot, Capt. Matthew W. Bancroft, 29, of Shasta, Calif.; co-pilot Capt. Daniel G. McCollum, 29, of Richland, S.C.; Gunnery Sgt. Stephen L. Bryson, 36, of Montgomery, Ala.; Staff Sgt. Scott N. Germosen, 37, of New York City; Sgt. Nathan P. Hays, 21, of Lincoln, Wash., Lance Cpl. Bryan P. Bertrand, 23, of Coos Bay, Ore.; and Sgt. Jeannette L. Winters, 25, of Du Page, Ill. All were based at the Marine Corps Air Station in Miramar, Calif.

President Bush said the crash was a reminder of "how serious the times are today."

"Our hearts and prayers go out to the families of the soldiers," Bush said at a fund-raiser for the re-election of his brother Jeb as governor of Florida. "But I want to remind them that the cause that we are now engaged in is just and noble. The cause is freedom and this nation will not rest until we've achieved our objective."

Pentagon sources told Fox News the military does not believe there was hostile fire involved in the crash. They believe a mechanical failure caused it — or perhaps a problem with geography or weather.

The plane crashed as it approached a military airfield called Shamsi in southwestern Pakistan. That air base is about 180 miles southwest of the Pakistani city of Quetta, according to U.S. Central Command.

The plane's flight originated in Jacobabad, Pakistan, and was making multiple stops.

Defense Secretary Donald H. Rumsfeld said he was saddened by the accident but did not know the circumstances of the crash or whether the KC-130 was on a refueling mission.

"I'm going to wait for the investigation to be completed," he said. "We've got some folks heading up there now.

"It is a tough, dangerous business over there," he added. "They're doing difficult things and they're doing them darned well, and it just breaks your heart."

A journalist, Saeed Malangzai, who lives about 40 miles from the crash site, told The Associated Press that the plane went down in the Lundi mountains in southern Balochistan province.

"Residents saw flames from the burning plane before it crashed into the Lundi mountains," Malangzai said.

The KC-130 is a $37 million plane routinely used by the Marine Corps for in-flight refueling of helicopters. It is also used for troop and cargo delivery, evacuation missions and special operations support. It carries a six-man crew of two pilots, a navigator, flight engineer, mechanic and loadmaster.

The only other fatal crash of a U.S. military aircraft during the war in Afghanistan, which began Oct. 7, was an Army Black Hawk helicopter that crashed in Pakistan on Oct. 19, killing two Army Rangers.

The Associated Press contributed to this report.