Civilians, mostly women and children, were killed last week in air raids by coalition forces in Afghanistan's Helmand province, an initial United Nations inquiry suggested.

On Thursday and Friday, as many as 18 civilians died in air strikes in Helmand's Sangin district, according to a statement released by the UN Assistance Mission in Afghanistan 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.

Afghan officials and local residents told Al Jazeera that the death toll was higher than 18.

"The jets arrived at around 3am (local time) on Friday and started bombing indiscriminately in a heavy residential area. I lost my aunt, two cousins and a nephew in the strikes," Mullah Fazal Ahmed told Al Jazeera.

"Six others in my family were wounded," said Ahmed, adding that the bombardment lasted for up to half an hour.

"Most of them who were pulled out of the rubble were bodies of women and children, as young as five-years-old."

US military officials said their aircraft 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, but let me tell you, no one has reached any final conclusion on this incident in Helmand," Brigadier General Charles Cleveland, a US army spokesman in Afghanistan told Al Jazeera.

"We all know that there is a fight against the Taliban going on in Sangin for the past 10 days, there are mortars being fired by the Taliban and Afghan forces are fighting them, so its not at all clear at the moment how these civilians were killed."

READ MORE: UN to probe US air raid that killed women and children

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 civilians were killed or injured in 2016, a figure highest in areas outside of Kabul.