Civilians, mostly women and children, were killed last week in air raids by NATO forces in Afghanistan's Helmand province, an initial UN inquiry suggested.On Thursday and Friday as many as 18 civilians died in air strikes in Helmand's Sangin district, according to a UN statement released on Sunday.The UN said the strikes had been conducted by "international military forces," but only US aircraft have been involved in recent coalition strikes, according to US military officials.Other US military officials said their aircraft have conducted around 30 air raids in Helmand in the past week. NATO's Resolute Support mission has initiated an inquiry into the incident."We are investigating the allegations and working diligently to determine whether civilians were killed or injured as a result of US air strikes," Brigadier General Charles Cleveland, a US army spokesperson in Afghanistan, said in a statement. READ MORE: UN to probe US air raid that killed women and children But Afghan officials and local residents have given a different toll of 22 civilians killed during the joint operation carried out by US and Afghan forces.The presidential envoy for security in Helmand, Jabar Qahraman, said the raid against Taliban fighters in the Sangin district killed 13 people from one family and nine from another.The NATO-led military mission has deployed hundreds of troops to Helmand in a bid to help Afghan security forces in their war against Taliban fighters.Civilian casualties from both American and Afghan air strikes increased dramatically last year, according to the UN's most recent report on threats to civilians. At least 891 civlians were killed or injured in 2016, a figure highest in areas outside of Kabul.