NANGARHAR: A suspected United States drone strike against the militant Islamic State (IS) group in Afghanistan killed 18 people on Wednesday, including a majority of militants but possibly civilians, as well, Afghan officials reported.

The drone strike occurred in the IS-stronghold Achin district of Nangarhar province, which lies near the Pak-Afghan border, killing 15 militants and three civilians, according to Mohammed Ali, police chief of Achin district.

"They were in a house to visit someone who had just come from the Haj pilgrimage," Ali said. "A drone targeted the house and killed most of them."

According to provincial police spokesperson Hazrat Hussain Mashriqiwal, several IS leaders had been killed. However, he has denied that there were any noncombatants among the victims.

A spokesman for the US military command in Kabul has confirmed that US forces conducted a "counter-terrorism" air strike in Achin, but chose to keep the details of the target classified.

"US Forces in Afghanistan take 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."

In order to determine which victims of the strike were militant and which were civilians, a complete investigation has to be carried out first, said Attaullah Khogyani, a spokesman for the provincial governor.

Civilian casualties resulting from US airstrikes have been a source of tension between the two allies as they have been fighting against militant opposition to the Afghan government since 2001.

Thousands of US troops remain in Afghanistan to help the government combat the Taliban, the dominant militant group in the country, and are conducting operations against S in Nangarhar.

According to US military reports, 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.