A suspected U.S. drone strike against Islamic State in Afghanistan killed 18 people on Wednesday, most of them militants but possibly including some civilians, Afghan officials said.
Civilians casualties in U.S. air strikes against Taliban and other militants in Afghanistan have long been a source of friction between the allies who have been fighting since 2001 to end militant opposition to the government in Kabul.
The strike in Nangarhar province, on the eastern border with Pakistan, killed 18 people, 15 of them militants and three civilians, said Mohammed Ali, police chief of Achin district where the attack occurred.
"They were in a house to visit someone who had just come from the Hajj pilgrimage," he said. "A drone targeted the house and killed most of them."
Provincial police spokesman Hazrat Hussain Mashriqiwal said several Islamic State leaders had been killed, but he denied there were any noncombatants among the victims.
Islamic State has enticed hundreds, perhaps thousands, of fighters in Afghanistan and Pakistan to join, and it holds some territory in Nangarhar, with Achin a stronghold.
But it has not been able to expand its influence in Afghanistan beyond a few districts and the Taliban remain the dominant Islamist force.
A spokesman for the U.S. military command in Kabul confirmed that U.S. forces conducted a "counter-terrorism" air strike in Achin, but would not discuss the details of the target.
"U.S. Forces — Afghanistan takes all allegations of civilian casualties very seriously," said Brigadier General Charles Cleveland.
"We are aware of some claims of Afghan casualties, and are currently reviewing all materials related to this strike. We are continuing to look into these allegations."
Determining which victims were civilians and which were militants would not be possible until an investigation was complete, said Attaullah Khogyani, a spokesman for the provincial governor.
Thousands of U.S. troops remain in Afghanistan, with many recent air strikes and special operations missions aimed at Islamic State in Nangarhar, as well as helping Afghan forces in their fight against the Taliban in various parts of the country.
Between January and August this year, American aircraft released more than 800 weapons and flew more than 3,500 close air support missions in Afghanistan, according to U.S. military reports.