A roadside bomb has killed three women and one man in southern Afghanistan.

Provincial officials say a tractor carrying the civilians drove over the bomb in the Maruf district of Kandahar province late Wednesday. The blast wounded three other people.

The United Nations mission in Afghanistan said last week that May was the deadliest month for Afghan civilians since it began keeping track in 2007.

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