The Army helicopter was taking part in an assault on suspected al-Qaida and Taliban fighters the Pentagon says were regrouping south of Gardez.

It would be 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.

Defense officials declined to give details, indicating they were unsure whether a search-and-rescue mission for the troops had been completed.

The incident came as the largest coalition force assembled so far in the campaign – some 1,500 from America, Afghanistan and at least six other nations – engaged an estimated several hundred al-Qaida and Taliban in intense firefights backed by air strikes.

Before the downing of the helicopter, the casualty toll of the operation started Friday stood at one American and three Afghans killed and an undisclosed number injured.

That raised to 20 the number of American military personnel who have died since the campaign began. The most deadly incident came Jan. 9 when six Marines were killed in the crash of a tanker plane into a mountain in Pakistan.

In the latest fighting, U.S. Chinook helicopters were ferrying in supplies to American and other troops in the hills following the start of the coalition ground attack in the area. In addition to allied Afghan fighters and U.S. Special Forces, troops from Australia, Canada, Denmark, France, Germany and Norway were participating.

The assault was believed to be the largest joint U.S.-Afghan military operation of the 5-month-old terrorism war. Pro-U.S. Afghan troops approached the hide-outs from three directions to isolate the fighters and prevent them from escaping.

Safi Ullah, a member of the Gardez town council, or shura, said the first stage of the offensive was designed to cut the road from Shah-e-Kot to trap al-Qaida and Taliban forces in the mountains. He said the plan also involved setting up checkpoints to prevent them from getting out.

Pakistan has closed its border with eastern Afghanistan and deployed extra army units and members of the Khasadar tribal militia to catch any who try to cross the frontier and filter into its Northwest Frontier Province.