KABUL, Afghanistan (AP) -- A powerful roadside bomb killed six policemen in western Afghanistan on Tuesday when their vehicle hit the explosives buried in the road, an Afghan official said.

Insurgents often use makeshift land mines to target both Afghan government officials and international forces in Afghanistan. The explosives often kill civilians as well, accounting for a large percentage of Afghan civilian casualties last year.

Tuesday's explosion in Herat province was so strong that the police truck was completely destroyed in the blast, District Police Chief Sher Agha said, adding that there were no survivors.

The policemen were guards on the Salma dam project and were on their way to Herat city, the provincial capital, when they hit the roadside bomb in Obey district, Agha said.

In recent months, the Taliban have launched a fierce offensive against the Afghan government they want to overthrow, unleashing a wave of assassinations and bombings. Insurgents have also been attacking police positions around the country, seeking to seize territory.

Their aim is to weaken the government ahead of the pullout of most international troops from the country by the end of next year.

Afghan security forces have been increasingly taking the lead in fighting insurgents, and this summer's fighting season is seen as a crucial test of their capabilities without foreign military fighting alongside them.