US troops opened fire on a bus carrying civilians in central Afghanistan, killing four passengers after their driver refused to stop. A spokesman for NATO's International Security Assistance Force said the bus was heading toward 20 troops on foot patrol on a highway in central Wardak province on Friday. The troops first fired warning rounds in the air to stop the vehicle, then shot into the engine block. The bus kept coming, so they opened fire on the vehicle in self-defense. The provincial governor and area residents confirmed that the troops were U.S. forces. At least 10 other passengers were wounded, Wardak Gov. Halim Fidai said. An ISAF statement said the wounded were evacuated to military hospitals.