Four U.S. soldiers were killed and three wounded in a bomb attack in Afghanistan on Sunday, the U.S. military said.

The three wounded men were hurt in secondary explosions as they tried to pull their fellow soldiers to safety after the first blast in Zabul province, in the south of the country.

U.S. forces have now suffered 47 deaths in combat in Afghanistan this year making it the worst period since they arrived to oust the Taliban in October 2001.

The unit attacked on Sunday in Dai Chopan district was involved in operations in support of the Sept. 18 parliamentary and provincial elections.

Sunday's casualty toll is the worst for U.S. forces in Afghanistan since 16 American servicemen were killed in June when their helicopter was shot down as they tried to rescue four trapped U.S. special forces soldiers.

Only one of the special forces soldiers survived.

PHOTO CAPTION

US soldiers patrol the streets of Kabul on August 18. (AFP)