The official spoke on condition of anonymity because the information had not yet been publicly released.
The Taliban claimed responsibility for the bombing in Shorabak district of Kandahar province near the Pakistan border.
Two Afghan policemen were also killed.
It was the deadliest day for coalition forces in Afghanistan since April 27, when an Afghan military pilot opened fire at Kabul airport and killed eight U.S. troops and an American civilian contractor.
The Associated Press contributed to this story