An Afghan soldier on guard at a joint base with U.S. troops shot dead two American servicemen and wounded two others as they slept, a provincial official said on Saturday.

Shahedullah Shahed, spokesman for the governor of Wardak province west of Kabul, said the shooting took place after a combined team of Afghan and U.S. forces had returned from a joint operation late on Friday.

"The Americans were in the middle of sleep when an Afghan soldier on duty opened fire on them," Shahed said.

"We have no clue as to why he shot them."