Rafi Behrozan, the spokesman for the Herat governor, said four others were wounded in the powerful blast in the Kushk Kuhna district of Herat province.

Two additional bombs found nearby on the same road were defused by Afghan policemen, he added.

"The explosion was very strong and the vehicle was destroyed. The majority of these aboard were killed," Behrozan said.

Also on Thursday, NATO said a coalition airstrike the previous day killed four Afghan soldiers after they were mistaken for militants.

Afghan Defense Ministry spokesman Gen. Mohammad Zahir Azimi said the soldiers had left their base in Helmand's Musa Qala district on Wednesday night when they came under fire from NATO planes.

NATO told the Afghan government that the coalition thought the men were militants, he added.