The United Nations in Afghanistan said on Thursday that an airstrike in the eastern province of Nangarhar near the Pakistan border killed at least 15 civilians and injured another 13.

In a statement, the U.N. Assistance Mission in Afghanistan called for an independent investigation into the incident, which took place in the Achin district early Wednesday.

It said that all the dead were men, and one of the injured is a boy. They were part of a crowd that gathered at the home of a tribal elder to welcome him back from his pilgrimage to Saudi Arabia to perform the Hajj ritual.

The U.S. military told The Associated Press on Wednesday that it had been conducting air operations against Daish group loyalists in Achin at the time.

Spokesman for U.S. forces in Afghanistan, Brig. Gen. Charles Cleveland said the claims of civilian casualties were being investigated.

In a statement Thursday, he said U.S. forces were working "with Afghan authorities to determine if there is cause for additional investigation."

The details in the UNAMA statement echoed those of local officials in the hours after the air strike.

Hazrat Hussain Mashreqiwal, the spokesman for the provincial police chief, said the airstrike had targeted Daish loyalists.

Achin and other districts in the province have been overrun by insurgents for the past year, with the Taliban and the Daish fighters often battling each other.

Afghan security forces backed by U.S. troops and air strikes have been conducting counter-terrorism operations in the area for months.

Afghan media have reported in recent days that Daish fighters have been returning to areas they had earlier been driven from.

The Daish group is a recent addition to Afghanistan's security woes. The Taliban have been fighting to overthrow the Kabul government since their own regime was ousted in a U.S. invasion in 2001.