The bus was traveling on the Kabul-Herat highway in the Adraskan district in western Herat province when it got close to a convoy of U.S. troops, Khaama Press said.
The reason for the attack was not reported.
Meanwhile, authorities in Cheghcharan city in western Ghor province said two NATO-led coalition troops were injured Tuesday when an Afghan security service member opened fire on them.
Provincial governor spokesman Abdul Hai Khatibi identified the two soldiers as Lithuanian. The conflict took place when an Afghan soldier asked Lithuanian troops to stop their vehicles, he said.
An Afghan soldier fired a rocket-propelled grenade at one of the vehicles, injuring the two soldiers, Khaama Press reported. One soldier is in critical condition.
The Afghan soldier who fired the grenade was arrested.