An Afghan official says a powerful roadside bomb has killed six policemen in the country's west.

The officers' vehicle hit the explosives planted in the road they were driving on Tuesday morning in Herat province.

District Police Chief Sher Agha says the explosion was so strong that the police truck was completely destroyed in the blast in Obey district. He says there were no survivors.

The policemen were guards of Salma dam project and were on their way to Herat city.

Insurgents often use makeshift land mines to target both Afghan government officials and international forces in Afghanistan. The explosives often kill civilians as well.