A suspected U.S. drone strike against Islamic State in Afghanistan killed at least 21 people on Wednesday, most of them militants but possibly including some civilians, Afghan officials said.

Civilians casualties in U.S. air strikes have long been a source of friction between the Afghan government and Western allies fighting the Taliban-led insurgency since 2001.

The strike in Nangarhar province, on the eastern border with Pakistan, killed 21 people, at least three of them civilians, and wounded another 11, according to Malem Mashooq, the governor of Achin district where the attack occurred.

One of the wounded told Reuters that the attack struck a house where people were sleeping after a gathering to welcome a local elder who had recently returned from the Hajj pilgrimage.

"I saw dead and wounded bodies everywhere,” Raghon Shinwari said, lying on a hospital bed in Jalalabad city.

Mohammed Ali, the Achin district police chief corroborated that account.

"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.

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.

Islamic State has attracted hundreds, perhaps thousands, of fighters in Afghanistan and Pakistan to join its ranks, and it holds some territory in Nangarhar, and Achin was a stronghold for the group.

But it has not been able to expand its influence in Afghanistan beyond a few districts and the Taliban remain the dominant militant force.

Thousands of U.S. troops remain in Afghanistan, with many recent air strikes and special operations missions aimed at Islamic State in Nangarhar, but they have also been helping Afghan forces fight Taliban elsewhere in 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.

-Reuters