KABUL, Afghanistan — An American national working for the United Nations in Afghanistan was killed and two others were wounded in a blast targeting a U.N. vehicle in Kabul on Sunday, officials said, underscoring a growing threat to aid workers.

The attack led U.N. agencies in Afghanistan to impose lockdowns on Monday, while other major international organizations restricted their movements as a precaution. Officials said the U.N. also canceled flights, even though many smaller aid organizations rely on air travel to get to parts of the country not accessible by roads increasingly plagued by fighting.

The latest attack further complicates humanitarian work at a time of dire need in the country and as a harsh winter looms. The U.N. estimates that about one million Afghans are “on the move,” having been either internally displaced by the intensifying conflict or because they are refugees in need of humanitarian assistance.

“It is with profound sadness that the U.N. family in Afghanistan confirms the killing tonight of a colleague, as well as injuries to two others, when a U.N.-marked vehicle was attacked in Kabul,” the local U.N. mission said in a statement late on Sunday.