The rugged mountains of eastern Afghanistan are hostile territory for U.S. helicopters, even without enemy forces trying to shoot them down.

Helicopter rotors don't work as well in the thin air at between 8,000 feet and 12,000 feet — roughly the same elevation as the ski resorts in Aspen, Colo. Clouds, ice, snow, sleet, and wind can make flying helicopters at high altitudes even more difficult.

"It is a difficult environment, not just for the human beings involved but for the helicopters," Defense Secretary Donald H. Rumsfeld said at a Pentagon news briefing Monday. "They're not really designed to fly at those altitudes."

Rumsfeld and other defense officials say they don't know whether altitude or weather made the two helicopters hit by enemy fire Monday more vulnerable. But they explained that flying into this combat zone is particularly tricky.

"As you operate higher with helicopters, the control effectiveness starts to diminish a little bit. So they're just against their operating limits," said Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff.

Two MH-47 Chinook helicopters took enemy fire Monday as they tried to drop special forces solders into the battle with suspected Al Qaeda forces in their mountain stronghold. Seven soldiers were killed.

At lower altitudes, the specially equipped helicopters can carry up to 44 special forces troops and their gear, using sophisticated radar systems and heads-up displays so pilots can fly low and fast in darkness. The Army special operations unit that flies the MH-47 helicopters calls itself the Nightstalkers.

In the challenging conditions of the mountains south of Gardez, Afghanistan, the helicopters probably will be loaded with fewer troops — perhaps even less fuel — to deal with the conditions, said Gen. Tommy Franks, the war's U.S. commander.

Franks said the military's helicopters and other tools were working well in the battlefield despite adverse conditions.

"When you're dealing in temperatures of 15 to 20 degrees, a little bit of sleep from time to time, at the altitudes that we're talking about, and in this very, very rough terrain, this is just very hard work for these soldiers who are up there doing it," Franks said.