The Taleban say they shot the Chinook down. The US concedes that "hostile fire" may have been the cause.

The Chinook was taking the soldiers to join operations against militants in the eastern province of Konar. Officials say there is still hope that some of those unaccounted for are alive.

US forces have been engaged in a number of operations in south-east Afghanistan against Taleban and al-Qaeda militants and their allies opposed to the Kabul government.

If the reports of hostile fire are confirmed, the Chinook would be the first US helicopter to be downed in such a way in Afghanistan.

US troops reached the crash site in a remote mountain valley late on Wednesday night. Bad weather had hampered the search which has been further complicated by the high altitude of the crash site.

Officials say the rugged mountain terrain had made it difficult for the rescue team to access the area.

A man who said he spoke for the Taleban phoned two international news agencies in Afghanistan with similar claims.

The incident came amid an escalating insurgency by the Taliban ahead of Afghanistan's landmark parliamentary and legislative elections in September.

More than 500 people, most of them militants, have died since the Taliban launched the offensive at the beginning of the year.

Such wave of Taliban violence culminating in the crash of a US helicopter probably hit by ground fire has brought Afghanistan to the verge of chaos and could compromise American operations in Iraq, analysts say.

Analysts said that if the United States could not defend its high-tech hardware against grassroots Taliban insurgents, questions would be raised about its ability to guarantee security ahead of landmark parliamentary polls in September.

"It is obvious the US military's campaign to eliminate or weaken the Taliban has significantly failed," said Riffat Hussain, an expert on Afghan affairs at Quaid-e-Azam University in the Pakistani capital Islamabad.