The UN voiced "grave concern" Sunday over the killing of at least 25 civilians in the escalating conflict in the southern opium-rich province of Helmand, most of them in US air strikes.

American bombardment in Sangin on Thursday and Friday killed 18 people, nearly all women and children, the United Nations said, as fierce fighting raised fears the district could fall to the Taliban.

A Taliban suicide bomber on Saturday struck Afghan army soldiers outside a bank in Helmand's capital Lashkar Gah, killing seven people including child vendors, in what the insurgents called revenge for the Sangin air strikes.

"The UN Assistance Mission in Afghanistan expresses its grave concern at the recent escalation of violence in Helmand province, which claimed the lives of at least 25 civilians, mostly women and children, and injured many more," the world body said.

"UNAMA reiterates the need for all parties to the conflict to strictly adhere to their obligations under international humanitarian law to take all feasible measures to protect civilians from harm."

NATO on Friday said it had launched an investigation into the air strikes, while Helmand's governor rejected reports that civilians had been killed in Sangin.

For years Helmand was the centrepiece of the Western military intervention in Afghanistan only for it to slip deeper into a quagmire of instability.

The US military stepped up air strikes in the province last week to aid Afghan forces who are struggling to stop the Taliban's territory-gobbling offensive.

The Taliban effectively control or contest 10 of the 14 districts in Helmand, the deadliest province for British and US troops over the past decade and blighted by a huge opium harvest that helps fund the insurgency.

Lashkar Gah -- one of the last government-held enclaves -- also risks falling to the Taliban's repeated ferocious assaults.

The intensified fighting in the province last year forced thousands of people to flee to Lashkar Gah from neighbouring districts.

UNAMA said 891 civilians were killed or injured in Helmand during 2016, the highest in the country outside of Kabul.