Spokesman Sediq Sediqqi said he had no further details about their nationalities or further casualties.
The convoy, escorted by security forces, was on its way from scenic Bamiyan province to the city of Herat in the west, he said.
Afghanistan remains locked in a violent insurgency and sees few foreign tourists outside of relatively peaceful areas like Bamiyan, which is home to the country's first national park.
Western embassies typically warn their citizens against all travel in Afghanistan, citing threats of attack and kidnapping.
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