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By Cassandra Vinograd

The United Nations said a U.S. drone strike killed at least 15 civilians in eastern Afghanistan, calling for a “prompt” investigation into the lethal incident.

The airstrike took place early Wednesday morning in the Achin district of Nangarhar province. At the time, the U.S. military acknowledged conducting a "counter-terrorism" airstrike in the area and said it was aware of claims of Afghan casualties.

An injured Afghan man receives treatment at a hospital following a suspected U.S. drone strike in the Achin district of Nangarhar province on Wednesday.NOORULLAH SHIRZADA / AFP - Getty Images

In a statement issued late Thursday, the U.N. Mission in Afghanistan said the strike killed at least 15 Afghan men and injured at least 13 others — including a boy — who'd gathered to celebrate a tribal elder's return from the Hajj. Students and a teacher were among the victims, it added.

"UNAMA reiterates the need for all parties to the conflict to adhere to their obligations under international humanitarian law," the statement said. "UNAMA calls on the government and international military forces to launch a prompt, independent, impartial, transparent, and effective investigation into this incident."

There was no immediate response from the U.S. military in Afghanistan.

The Achin incident took place almost exactly a year after a U.S. airstrike on a hospital in the Afghan city of Kunduz killed at least 42 people.