At least 15 Afghan forces have been killed when US warplanes conducted an airstrike in Afghanistan's Helmand province.

The US military described the Friday attack as a friendly-fire incident, promising an investigation.

"We would like to express our deepest condolences to the families affected by this unfortunate incident," the US military said in a statement.

It said the airstrike hit a compound in Helmand's Gereshk district, where the Afghan troops had gathered. The Afghan government had already been notified about the air raid, according to the statement.

The statement came short of announcing the number of Afghans who lost their lives in the strike.

Pajhwok Afghan News, however, quoted a security official as saying that 15 police, including two commanders, were killed in the attack.

The US commander in Afghanistan John Nicholson (R) talks with soldiers ahead of a handover ceremony at Leatherneck Camp in Lashkar Gah in the Afghan province of Helmand on April 29, 2017. (Photo by AFP)

Every so often, US air raids kill both Afghan soldiers and civilians across the country.

Airstrikes by US warplanes have also significantly increased in recent months against the purported positions of the Taliban militant group and other terror outfits in the crisis-hit country. US President Donald Trump is also considering requests from military commanders for thousands more international troops.

In May, American officials said the US military needed between 3,000 and 5,000 additional troops, including hundreds of Special Operations forces, in Afghanistan.

The United States currently has about 8,400 soldiers in Afghanistan with another 5,000 troops from NATO allies.

Despite the presence of thousands of foreign boots on the ground, Afghanistan has been rocked by an increase in terrorist attacks, some of them carried out by the Daesh Takfiri terrorist group mainly active in Nangarhar province.

The country has already been torn apart by decades of Taliban-led militancy and the 2001 invasion by the United States and its allies.