Concern is mounting over a team of US special forces on a reconnaissance mission who have been missing in eastern Afghanistan since Tuesday.
Airborne forces had been sent in to help the ground troops
A helicopter which was sent to pick them up was shot down on Tuesday, killing 16 servicemen on board.
US forces hunting the missing ground team say they are using all available means, including unmanned surveillance aircraft and numerous ground troops.
But their efforts have been severely hampered by bad weather.
Torrential downpours hit much of the region late on Friday, making the hard mountain terrain in Konar province even tougher going for search teams on the ground.
It has also forced the suspension of most helicopter flights.
People claiming to speak for Taleban militants say they have captured one American on the ground, but have provided no proof.
It is unclear whether any captured American would have been from the helicopter or the missing team on the ground. However, the US initially said 17 men were on board the downed helicopter before reporting that 16 bodies had been found.
US officials have refused to comment on the Taleban statement that an American is being held.
Military officials told the BBC's Andrew North, who is at the main US base in eastern Afghanistan. that they had "several indications" that the troops on the ground were still alive.
However, they would not say what those indications were.
They also refused to say why the unit had not yet been brought to safety.
In Kabul, US military spokesman Col Jim Yonts said: "We do not have eyes on them right now, but there is no reason to believe that they are dead."
Col Yonts would not confirm or deny an earlier claim by the Taleban that its fighters had killed seven US "spies" before the MH-47 helicopter was shot down.
A number of Afghan guides working with the US military are also missing.
The Pentagon said eight soldiers from airborne special forces units and eight navy Seal commandos were killed in Tuesday's crash.
The downing of the helicopter was the worst single US combat-related loss since invading Afghanistan in 2001.
The Chinook was the first US helicopter to be shot down in Afghanistan since March 2002.
The Taleban say they shot down the aircraft.
Officials say they are still investigating the exact reason why the helicopter came down.
Our correspondent says it seems to have crashed at some distance from the point where it is believed to have been hit by at least one unguided rocket-propelled grenade.
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
Speaking in Washington, Lt-Gen James Conway, director of operations for the joint chiefs of staff, said: "Indications are that it was an RPG, which is a pretty lucky shot, honestly, against a moving helicopter."
Clashes between US-led forces and suspected Taleban and other militants opposed to the Kabul government have been on the increase this year in eastern and southern Afghanistan.
Nato plans to take over security across southern Afghanistan next year, gradually relieving the American force.