In addition to the three deaths, one other American soldier was injured when an Afghan solider open fired on them in the Nangarhar province in eastern Afghanistan.

Attahullah Khogyani, a spokesman for the provincial government, said the attack took place in the province's Achin district where Afghan and American forces have been fighting side-by-side against militants from the Taliban and the so-called "Islamic State."

Khogyani said that the Afghan soldier had been killed during the attack.

Though initial reports said two service members had been killed, US officials later upped the number of fatalities to three. 

The US military initially had issued a short statement saying it was "aware of an incident in eastern Afghanistan." 

A White House spokesperson traveling with Donald Trump in New Jeresey said the US president was aware of and following the situation. 

The Trump administration is considering sending more troops to the country.

The Taliban claimed responsibility for the attack. Its spokesman Zabihullah Kujahid said online that the solider had been an "infiltrator" who had killed four American soldiers. The group is known to inflate the number of its victims.

However, Khogyani said "The cause of the shooting is not clear." He added that an investigation was already underway.

Read: Ashraf Ghani offers Taliban 'last chance' for peace

'Green-on-blue' attacks decreasing

It is not the first time that a local solider has fired on its international partners in what is known as a "green-on-blue" shooting.

In a similar incident in March 2017, an Afghan solider wounded three American soldiers when he opened fire on them at a base in Afghanistan's southern Helmand province. Officials have stressed that such attacks are rooted in personal and cultural misunderstandings rather than inside insurgency schemes. The killings have affected trust between the local and international partner forces, even though they have decreased in recent years.

American and Afghan troops have been fighting IS and Taliban militants in Nangarhar's Achin district since January 2015. In mid-April of this year, the US dropped the "Mother Of All Bombs" on IS fighters in the area. It was the largest non-nuclear bombed ever used by the US in combat.

cmb/rc (AP, dpa, AFP, Reuters)