Three US soldiers were killed and another was wounded in eastern Afghanistan on Saturday, confirmed the US Defense Department.

An investigation is underway, it said in a brief statement, without giving further information. 

An Afghan commando had opened fire on the US troops during a joint operation in the Achin district of Nangarhar Province, said Attaullah Khogyani, a spokesman for the Nangarhar provincial governor.

The shooter was killed in the incident, Khogyani added.

A US official has also said the incident was a "green-on-blue" attack, which means an Afghan policeman or soldier firing on coalition forces in Afghanistan, reported Reuters.

Green-on-blue attacks were a major problem several years ago, but now occur less frequently after security measures were improved and the number of foreign troops in the country fell sharply.

The latest such attack happened as US President Donald Trump was reportedly mulling over increasing troop levels for Afghanistan.

In March, an Afghan soldier was killed after he opened fire on foreign forces at a base in Helmand Province, wounding three US soldiers.

(Source: agencies)