By Frank Gardner
BBC security correspondent, Bagram airbase, Afghanistan
In the remote mountains of Afghanistan an unusual mission is underway.
US troops have been beefing up their presence in the remote region
Large numbers of US troops have dropped in by helicopter to a place that has never seen Americans before.
US intelligence believes that the wild region of Nuristan and Kunar is harbouring militants linked to al-Qaeda and other groups.
Operation Mountain Resolve is aimed at destroying them and their bases. The US military has found it hard to locate al-Qaeda.
Now, it says, its troops have fought some gun battles up here but that their enemy has been running away.
The US admits this is not an easy operation.
There are frequent clashes between US-led forces and rebel fighters
"This is some of the toughest terrain in Afghanistan," says Major Rodney Davis.
"It's some of the most dangerous terrain we've operated in since we've been in Afghanistan. Needless to say it has an impact on the soldiers involved in the operation."
US war planes are also involved, called in by the troops on the ground.
The air force considers this operation such a high priority, it has even diverted some of its planes from Iraq.
They have been flying ahead of the ground troops to warn them of what lies ahead.
Hunting 'elusive enemy'
We found this pilot as she was returning from a mission over Nuristan.
"We were just up there last night and they're moving," says Captain Jennifer Short.
"From last night just to today, they've made some progress and they sound good. So I think it must be going pretty well."
However, she would not be drawn on details.
For several days now this airbase at Bagram has been sending helicopters and attack aircraft up into the mountains of Nuristan.
They are hunting an elusive enemy.
But in the backs of everybody's minds is the possibility - just the possibility - that Osama Bin Laden himself could be up there.