A roadside bomb has hit a bus in southwestern Afghanistan, killing at least 13 people.

Police say the late Thursday attack in the Khash Rod district of Nimroz province was the work of the Taliban.

At least 18 people were wounded in the roadside bombing.

NATO joined Afghan President Hamid Karzai on Friday in condemning the attack.

Violence in Afghanistan has hit the worst levels since the U.S.-led invasion in 2001.

May was the deadliest month for Afghan civilians since the United Nations mission began compiling statistics four years ago.

The U.N. said 368 civilians were killed last month, with the majority of casualties caused by the Taliban and other militants.