The Pentagon says the mission of US Marines in Afghanistan is to cut off escape routes for Taliban and al-Qaeda leaders.
The soldiers will also pinpoint targets for airstrikes and make quick strikes when the chance arises.
US Marines strengthened a bridgehead in southern Afghanistan today to advance their campaign to find Saudi-born militant Osama bin Laden.
Defense Secretary Mr Donald Rumsfeld said more than 1,000 marines should be at the desert airstrip near Kandahar in days as planes and helicopters flew in at regular intervals with soldiers, armour and supplies.
Soldiers on the ground called in Cobra attack helicopters to fire on a convoy of 15 vehicles heading toward the airstrip last night. Several vehicles were destroyed.
Local Afghans identified the facility as Dolangi airstrip, about 55 miles southwest of Kandahar. They said it was rebuilt and used by bin Laden.
Five US soldiers seriously injured by a stray US bomb have been evacuated to a medical centre in Germany.
The Pentagon says a US attack aircraft mistakenly dropped a bomb on them near Mazar-e-Sharif in northern Afghanistan. A Pentagon statement says the injuries are not life-threatening.