The US military says hostile fire probably brought down a helicopter which crashed in eastern Afghanistan.
The Chinook is regularly used as a troop carrier in Afghanistan
A search is under way for the 17 crew members of the Chinook helicopter in the province of Konar. It is not known if there are any survivors.
The hardline Taleban militia - driven from power by US forces in 2001 - has claimed it downed the helicopter.
US forces have been leading an assault on Taleban and al-Qaeda guerrillas opposed to the Kabul government.
A US military spokeswoman said "initial reports indicate the crash may have been caused by hostile fire".
"The status of the service members is unknown at this time," she said.
Twin-engined aircraft with two, three-bladed rotors
Used to move soldiers, weapons and supplies
Can carry 54 troops or 25,000 lbs (11,340 kg) of freight - more than its own weight
Crew of four
US forces will be wary of using more helicopters in the rescue effort for fear they too could be attacked, says the BBC's Andrew North, who is at the US base where the search is being co-ordinated.
The rescue operation has been further complicated by the high altitude of the crash site, a remote, moutainous area west of the town of Asadabad.
If the reports of hostile fire are confirmed, the Chinook will become the first US helicopter to be downed in such a way in Afghanistan since March 2002.
A sandstorm was blamed for a US helicopter crash in April that killed 18 soldiers - the single heaviest loss of life for US troops since they entered Afghanistan in 2001.
In all at least 29 US soldiers have died in Afghanistan this year.
The twin-engined aircraft that crashed on Tuesday had been sent to support troops on the ground.
"The helicopter was transporting forces into the area as part of Operation Red Wing, which is part of the enduring fight to defeat al-Qaeda militants and deny them influence in Konar province," a military statement said.
US and Afghan troops have sealed off the crash site to block enemy movement around it and US aircraft are flying overhead, the statement added.
The governor of Konar province, Asadullah Waffa, told the BBC the helicopter had been brought down by a rocket.
He said the attack was the work of well-funded militants who had entered Afghanistan planning to spread chaos before September's parliamentary elections.
His security forces recently arrested two such militants posing as cameramen, he said.
Upsurge in fighting
A man claiming to be a spokesman for the Taleban told the BBC its supporters had shot the helicopter down.
He said he had video of the crash and its aftermath.
A different man who said he spoke for the Taleban phoned two international news agencies in Afghanistan with similar claims.
At least one of the phone calls came before the US officially released news of the crash.
US military forces regularly come under attack in Konar province.
There has recently been an increase in fighting between US-led troops and militants.
An estimated 450 people have died in Afghan violence over the last three months, most of them militants.