The attack, which took place in the Qarabagh district of the insurgency-wracked Ghazni province, also injured seven civilians, deputy provincial police chief Asadullah Insafi told AFP.
"We think that the target of the attack was commander Dawlat Khan, who was martyred along with three of his bodyguards, and three civilians in this attack," Insafi said.
Khan led a group of militiamen who were taking on the Taliban in several villages and planned to join the ranks of the Afghan Local Police (ALP), Insafi added.
Deputy provincial governor Mohammad Ali Ahmadi confirmed the account saying that seven civilians were also "seriously" wounded in the blast which he described as "huge".
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Taliban insurgents have carried out a campaign of assassinations of pro-government figures and warned Afghans to distance themselves from the government.
Yesterday the Taliban -- reportedly helped by infiltrators -- captured at least 12 Afghan policemen from their check-post in the southern province of Zabul and took them to unknown location.
Government forces have formally taken responsibility for security from US-led NATO troops who are due to leave the country next year. But there are concerns about their ability to stand against the Taliban unaided.