US bombers on Monday pounded al-Qaida and Taliban positions in the eastern mountains of Afghanistan after a 15-hundred-strong coalition ground attack failed to dislodge the well-armed fighters.
During the assault, the Pentagon said a US army helicopter was shot down, with at least six Americans killed.
It is the first American aircraft taken down by enemy fire in the war, following accidental crashes of other craft since the anti-terror campaign opened in October.
The assault, which began with bombing raids late on Friday, is believed to be the largest joint military operation of the five-month-old US-led campaign against terrorism.
The bombardments sent thick, black plumes of smoke above the snowcapped peaks and bombers streaked overhead.
Afghan officials said as many as five-thousand al-Qaida and Taliban fighters were regrouping in eastern Afghanistan and just over the border in Pakistan, urging the faithful to wage holy war against US forces.