Five U.S. soldiers were killed by a bomb in the southern Afghan province of Kandahar on Saturday, U.S. authorities said, capping off one of the bloodiest weeks for international forces this year.

The attack underscored the dangers faced by the NATO-led International Security Assistance Force (ISAF), even as they hand over much of the fighting to the Afghans before a planned departure next year.

Nineteen U.S. personnel have been killed in the last week in three air crashes and Saturday's bombing. Three British soldiers were also killed on Tuesday by a roadside bomb in the southern province of Helmand.

ISAF said on its website that the five soldiers were killed by an improvised explosive device in southern Afghanistan. A U.S. forces spokesman in the region and a Pentagon official said the five dead were U.S. troops.

Kandahar authorities said the soldiers' vehicle struck an IED in Maiwand district, which borders Helmand province to the west and is considered one of the most volatile of Kandahar's districts.

Last month, nine Afghan men working for a mine clearance organisation were taken captive by Taliban fighters there, though they were later released.