The United Nations has denounced the death of 15 civilians, including a child, in a US air strike against militants from the so-called Islamic State in eastern Afghanistan, calling for an independent investigation into the killings.

The drone attack occurred on Wednesday in Achin district, a hotbed of IS insurgents in Nangarhar province near the border with Pakistan, as villagers gathered to welcome a tribal elder who had returned from the hajj pilgrimage.

"UNAMA condemns the killing of at least 15 civilian men and the injuring of at least 13 others, including at least one boy, in the strike," the United Nations Assistance Mission in Afghanistan said last night.

"Civilian victims of the strike included students and a teacher, as well as members of families considered to be pro-government."

Afghan authorities had previously put the civilian death toll at between three and 13.

The American military acknowledged it had conducted the "counter-terrorism airstrike" on Wednesday, adding it was still probing the incident.

"United States Forces - Afghanistan takes all allegations of civilian casualties very seriously," the US military said in a statement, highlighting the challenge of targeting IS insurgents making steady inroads in Nangarhar.

"Daesh [another name for IS] is killing innocent Afghan men, women, and children. They continue to put innocent lives at risk by deliberately surrounding themselves with civilians and dressing in female attire," it said.

Islamic State first emerged in Afghanistan in late 2014 and has since violently challenged the much larger Afghan Taliban movement in parts of the country's east.

But the fighters have steadily lost territory in recent months because of stepped-up US airstrikes and a ground campaign by Afghan forces in Nangarhar.

Civilian and military casualties caused by NATO forces have been one of the most contentious issues in the 15-year campaign against the insurgents, prompting harsh public and government criticism.

A US air strike killed eight Afghan policemen earlier this month in the southern province of Uruzgan in the first apparent "friendly fire" incident since American forces were given greater powers to strike at insurgents in June.

The new authority gave the US-led NATO troops greater latitude to order air strikes in support of Afghan troops.