Enemy forces have suffered a much higher casualty rate since March 1 (U.S. date), when coalition forces began attacking an Al Qaeda-Taliban enclave south of the city of Gardez, Rumsfeld told Pentagon reporters. More than 1,000 U.S. troops are involved in the operation, as are German, Australian, Danish, Canadian, French and allied Afghan forces. 

The American deaths occurred during two separate incidents involving MH-47 helicopters. In the first, the aircraft was getting ready to land. A rocket-propelled grenade hit the chopper but did not explode. The pilot lost control for a time, but recovered and landed. One serviceman died in that incident. 

In the second incident, an MH-47 drew small arms fire and made a hard landing in enemy territory. A firefight ensued as other U.S. troops arrived to rescue the crew. A total of eight died in that incident. Officials said it's unclear whether the U.S. deaths occurred in the initial attack or the firefight. 

Earlier today, Pentagon spokeswoman Victoria Clarke told reporters that three dozen U.S. troops had been wounded so far in the fighting. Rumsfeld noted those wounded have been evacuated for medical attention, and he offered his condolences to the families of the dead and wounded. 

Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, said much of the battle area is "like the Rocky Mountains in the middle of winter," with elevations from 8,000 to 11,000 feet above sea level. The environment is cold and difficult for the troops and their machines, Myers said. The thin air, he noted, reduces the lifting ability of the helicopters. 

Rumsfeld praised the efforts of U.S., coalition and friendly Afghan forces. He said military operations would continue until remaining Al Qaeda and Taliban forces either surrender or are killed. 

"The choice is theirs," he emphasized. 

The secretary noted that coalition and Afghan ground forces are positioned "to check any large-scale effort to escape."