The United Nations has condemned the killing of four Afghan mine clearance workers who were kidnapped in the southwest part of the country last week.

U.N. officials say 31 workers were ambushed and kidnapped by unidentified gunmen while driving to a worksite in the Bala Buluk district on Wednesday.

Afghan officials say at least four of the de-miners were killed. More than two dozen workers were released on Monday.

The U.N. says landmines planted during the past two decades of conflict kill or injure an average of 40 people every month in Afghanistan.