Up to 23 civilians were killed in a US airstrike in Afghanistan, the UN has said.

A preliminary investigation by the UN mission in the country (UNAMA) has found the 'vast majority' of those killed were women and children.

Three other civilians were injured in the strike in the Helmand province on Tuesday, during an operation by pro-government forces targeting Taliban in the area.

In a statement UNAMA said: "The Mission is actively working to verify information indicating up to 10 children were killed along with eight women, and three children were injured, including an eight-year-old boy.

"UNAMA reminds all parties to the conflict to uphold their obligations to protect civilians from harm, and holds that all parties to the conflict must strictly adhere to their obligations under international humanitarian law to take all feasible measures to protect civilians."

It adds there are indications that "relevant authorities have initiated investigations into the civilian casualty reports".

The US military had previously confirmed its aircraft carried out the strike, saying it was called in by the ground force in 'self-defence'.

Earlier this week, a military spokesperson told The Washington Post: "At the time of the strike, the ground force was unaware of any civilians in or around the compound; they only knew that the Taliban were using the building as a fighting position."

According to UNAMA, there has been a 'sharp increase' in civilian casualties from aerial attacks in Afghanistan this year, with 649 deaths recorded in the first nine months of the year.

That number is more than any year since the organisation started 'systematic civilian casualty documentation' in 2009.