The US military is building a new airbase in south-eastern Afghanistan, in a remote desert area near the border with Pakistan.
By Andrew North
BBC correspondent in south-east Afghanistan
US commanders say the airstrip, in Paktika province, will be capable of taking large transport aircraft.
The move is aimed at improving security in the region.
It should also allow US forces to move in more ground troops to combat remnants of the Taleban and al-Qaeda, who remain active in the area.
Built in four days
The operation began in the dark, with five giant US air force planes dropping tons of construction equipment onto the desert plateau where the airstrip is being built.
Under the terms of being with the unit of US soldiers, the exact location cannot be given.
Soldiers on the ground guided in the planes, which had flown all the way from Germany.
Work began soon afterwards, bulldozing the hard, rocky scrub.
Taleban militants remain active in the area and just across the border
In less than four days' time, the troops of the 27th Combat Engineering Battalion aim to have created an airstrip capable of taking C-130 transport aircraft.
Even larger aircraft will eventually be able to land here.
US commanders will not say exactly how they intend to use the base, but it will allow them to move far larger forces into this region more quickly.
This is an area where suspected Taleban militants remain active and just across the border is the Pakistani tribal area of Waziristan, where until recently Islamic militants were battling the Pakistani military.
The new American presence on this side of the border has so far attracted little attention, apart from a few trucks on a nearby road.
But US forces are keeping a close watch on the area, with aircraft patrolling as well as troops on the ground.