A US drone strike has killed at least 21 people in the eastern Nangarhar province, with Afghan sources saying that more than a dozen of them were civilians.

Esmatullah Shinwari, a Nangarhar lawmaker, told the Associated Press that a crowd had gathered at the tribal leader’s house in the Achin district to welcome home his return from the annual Hajj pilgrimage to Saudi Arabia when the drone struck.

At least 12 people were wounded in the attack, including the tribal leader, he said.

Unnamed Afghan sources have said at least 13 of the victims were civilians.

The spokesman for the US military in Afghanistan, Brigadier General Charles Cleveland, confirmed that a “counter-terrorism airstrike” had been conducted in Achin district early Wednesday.

“US forces did conduct one counter-terrorism airstrike in Achin district, Nangarhar Province, Sept. 28, and for operational security reasons, we do not discuss the details of counter-terror operations,” he said in a statement.

“We are aware of some claims of Afghan casualties,” he said, adding that investigations were underway.

Local Afghan sources said the US drone attack left at least 25 civilian casualties.

The people of Afghanistan have been suffering from violence and insecurity since the US and its allies invaded the country as part of Washington’s so-called war on terror in 2001. Many parts of the country still remain plagued by militancy despite the presence of foreign troops. The military invasion removed Taliban, but militants still push to wrest control over the war-ravaged country.