Abdul Wadood Wahidi, spokesman for the provincial governor, said the bombing late yesterday targeted a meeting of criminal groups that in the past have clashed with both national security forces and Taliban insurgents.
He said seven others were wounded by the blast in Khan Abad district.
Taliban spokesman Zabihullah Mujahid claimed the attack.
The Taliban have stepped up attacks across the country since US and NATO forces shifted from a combat to a support and training role at the end of last year.