NATO troops fired on a civilian bus Friday in central Afghanistan that refused warnings to stop, killing four passengers, a spokesman for NATO's International Security Assistance Force said. The troops were on a foot patrol near a major highway when the bus started veering toward them, the force said in a statement. The bus kept coming after the troops fired warning shots in the air and after they fired into the engine block, so they fired directly on the bus in self-defense, the statement said. The governor of central Wardak province had previously said the bus was caught in the crossfire of a battle between US forces and insurgents. The NATO force, which does not identify the nationality of troops in such incidents, said the patrol did come under fire Friday morning, but that this occurred separately from the bus incident. Wardak Gov. Halim Fidai said at least 10 other passengers were wounded. The ISAF statement said the wounded were evacuated to military hospitals.