Three American soldiers were killed and another wounded in Afghanistan when an Afghan soldier turned his gun on them in a so-called "green on blue" attack.

A US source confirming the incident although details are scarce. The Pentagon only stating they've opened an investigation into a shooting.

But the governor's office in Nangarhar province says it took place there, where American special forces have been fighting alongside Afghans against Islamic State and the Taliban.

The Taliban say the Afghan soldier was one of their own. He, too, was killed in the incident.

Incidents of Afghan forces attacking coalition troops were major problem several years ago, but has died down with the addition of new security measures for the forces.

The number of US soldiers in the country has also fallen - a high of about 100,000 in 2010 to 8,000 today.

This latest attack comes as the White House considers increasing those numbers again - discussions thought to center on an additional 3,000 to 5,000 US and allied forces.

The Pentagon estimates the Afghan government has lost control of about 40 percent of the country.