An injured Afghan youth receives treatment at a hospital following a U.S. unmanned aircraft strike in the Achin district of Nangarhar province on September 28, 2016. (Noorullah Shirzada/AFP/Getty Images)

United Nations officials Thursday condemned an airstrike by an unmanned U.S. military aircraft a day earlier that they said killed 15 civilians and wounded at least 12 in the insurgent-plagued eastern Afghan province of Nangahar. They called for a complete investigation. 

The early morning attack targeted a residential compound in the volatile Achin district, near the border of Pakistan, which U.S. military officials said they believed was being used by fighters for the Islamic State militant group, widely known in Afghanistan as Daesh.  

However, local leaders and legislators said the victims were all civilians, including children and a teacher, who had gathered at a guesthouse to welcome home a tribal leader who had just returned from the Hajj pilgrimage to Mecca. All were said to have been sleeping when the strike hit. 

Afghan police said the strike had targeted Islamic State loyalists, and U.N. officials quoted government sources as reporting that several Taliban or Daesh militants also had been killed.

In a statement Thursday, officials of the U.N. Assistance Mission to Afghanistan called for Afghan and foreign military forces to launch a “prompt, independent, impartial” and effective investigation. They also stressed “the need for all parties to the conflict to adhere to their obligations under international humanitarian law.”

U.S. military officials confirmed Wednesday that they had conducted a “counterterrorism airstrike” in that area and were investigating reports of Afghan casualties. They did not release details but said they were “reviewing all materials related to the strike.”

In a statement, the U.S. officials said they take “all allegations of civilian casualties very seriously” but added that “Daesh 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.”

They said they would work with Afghan officials to determine whether there was “need for further investigation.”

They also noted that Daesh has been active in Nangahar since 2015, particularly in Achin, and that it uses the area to “train, equip, disseminate propaganda, and expand their control over innocent Afghans.” 

U.S. military forces have been working closely with Afghan security forces to drive Daesh from the area. A U.S. airstrike in July killed the senior Daesh leader in the region.