An Afghan family returning to their home after fleeing a possible military operation struck a roadside bomb Saturday in the country’s south, killing at least 12 of them and wounding eight, authorities said.

The blast struck the family’s van in Helmand province’s Marjah district, largely a Taliban stronghold. At least 12 died in the blast, though the death toll could rise, said Nabil Jan Mullahkhail, the provincial police chief.

The family fled the district in recent days after hearing a government offensive could take place there, relative Mohammad Salih said. Salih said the death toll likely was higher, as he believed as many as 21 of his relatives had been killed in the blast.

No one immediately claimed responsibility for the blast. The Taliban routinely plant roadside bombs to target Afghan security forces, though the bombs often explode as civilian vehicles pass by, killing them instead.

In February, the United Nations Assistance Mission in Afghanistan documented 10,548 civilian casualties in 2014, the highest number in a single year since 2009. They include 3,699 civilian deaths, up 25 percent from 2013.

The U.N. said at the time that the Taliban and other insurgents were responsible for 72 percent of all civilian casualties, with government forces and foreign troops responsible for just 14 percent. The U.N. report attributed the rise in casualties to intensified ground fighting, in which weapons like mortars, rockets and grenades are used in populated areas, sometimes indiscriminately.

U.S. and NATO troops pulled back from volatile areas last year, handing security responsibility over to Afghan forces and officially concluding their combat mission at the end of the year. In the time since, local Afghan security forces have been increasingly targeted in insurgent attacks.

Report by Associated Press.