The fireball that erupted in the crash of a U.S. military aircraft in Pakistan, killing all seven Marines aboard, apparently was created by the fuel-laden plane's impact into a mountain ridge rather than by hostile fire, Defense Secretary Donald H. Rumsfeld said Thursday.

"There is no evidence that it was anything other than an aircraft crash," Rumsfeld told reporters.

Witnesses had reported seeing flames shooting from the KC-130 before it slammed into the mountainside.

"There are always going to be eyewitnesses that have different impressions as to what they saw," Rumsfeld said.

He said efforts to recover the bodies were underway but were hampered by the difficult terrain.

Navy Lt. Cmdr. Matthew Klee, a spokesman at U.S. Central Command headquarters in Tampa, Fla., said officials had no confirmation that a crash investigation team en route to the site had arrived Thursday. A search-and-rescue team reached the site by foot on Wednesday but found no bodies, other officials said.

Rumsfeld said he was uncertain whether any bodies had been recovered.

"It's a very difficult area," he said at a joint news conference with his Australian counterpart, Robert Hill.

Rumsfeld and Hill both expressed sympathy for the family and friends of the victims.

"Their deaths, along with that of the U.S. Special Forces soldier last week, underscore the fact that the mission in Afghanistan remains difficult and remains dangerous," Rumsfeld said. "We have said that repeatedly. It is a fact, and it will continue to be a fact during the weeks and months ahead."

President Bush, in a visit to the Pentagon to sign a defense spending bill, praised the dead Marines as brave guardians of American freedom.

"In our quest to save civilization, there are enormous sacrifices, and there's no more greater sacrifice than the loss of life," he said. The final words of the president's address to the uniformed and civilian Pentagon workers in his audience were, "Stay on course; find the enemy."

Klee said there was no indication of what caused the plane to crash. It was the worst U.S. casualty toll of the Afghanistan campaign, and included the first woman killed since U.S.-led Afghan bombing began in early October.

Rumsfeld said the KC-130, which is used to refuel helicopters and other aircraft in flight, was carrying "bladders," or containers, of fuel when it crashed. This created the fireball, he said.

"The fireball occurred, according to the best evidence we have, as it hit the ground, not before it hit the ground," he said.

In Kandahar, Afghanistan, where the Marines have established a base, spokesman 1st Lt. John Jarvis said the plane was on its final approach to the Shamsi airfield. It had been on the first of a scheduled four stops, so it probably had an almost full cargo of fuel.

A military team headed from the Marine base at Kandahar to Shamsi to investigate, Jarvis said, but the site is not accessible by vehicle.

The Pentagon identified the seven Marines who were killed as: pilot Capt. Matthew W. Bancroft, 29, of Redding, Calif.; co-pilot Capt. Daniel G. McCollum, 29, of Irmo, 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 Wilbur, Wash.; Lance Cpl. Bryan P. Bertrand, 23, of Coos Bay, Ore.; and Sgt. Jeannette L. Winters, 25, of Gary, Ind. All were based at the Marine Corps Air Station in Miramar, Calif.