(CNN) At least 26 civilians, most of them women and children, were killed in Afghanistan's Helmand province last week, most of them victims of airstrikes conducted by international forces, a UN relief organization said Sunday.

The UN Assistance Mission in Afghanistan said it "expresses its grave concern" over the deaths.

Eighteen of the civilians, "nearly all women and children," were killed in Helmand's Sangin District amid airstrikes targeting "anti-government elements," UNAMA said.

The NATO-led Resolute Support Mission has begun an investigation into the incident, UNAMA said.

NATO bills the mission as an effort "to train, advise and assist the Afghan security forces and institutions." It consists of more than 13,000 troops, more than half of which come from the United States

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