At least 11 civilians were killed when a roadside bomb ripped through their vehicle in the southern Afghan province of Helmand, officials said on Saturday.

The blast hit a van travelling from the remote Nawa district to the provincial capital Lashkar Gah on Friday, government spokesman Omar Zhwak told AFP news agency.

"The blast was powerful and all those on board the van were killed," Zhwak said, adding that officials were trying to determine whether any women or children were among the victims.

Helmand police chief Agha Noor Kentoz blamed the Taliban for the blast, saying the road had been mined to target Afghan security forces who frequently use it.

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A Taliban spokesman was not immediately reachable for comment, but roadside bombs have been the group's weapon of choice in the war against foreign and Afghan security forces.

Planted improvised explosive devices are also increasingly killing and wounding civilians.

Most of Helmand, the biggest poppy-growing province in the country, is already estimated to be under Taliban control with Lashkar Gah - one of the last government-held enclaves - also at risk of falling to the armed group.

About 11,500 civilians, of whom a third were children, were killed or wounded in Afghanistan in 2016, according to the United Nations.

Afghan security forces now control less than 60 percent of the country, according to US estimates, with the Taliban holding about 10 percent and the remainder contested between various armed groups.