Two Afghan men wearing military uniforms have shot dead two US soldiers, in the first suspected "insider attack" of the year as NATO troops withdraw after 13 years of fighting Taliban militants.

Incidents in which Afghan forces turn their guns on their allies have killed scores of US-led troops, breeding fierce mistrust and undermining efforts to train up local forces to secure the country.

"Two International Security Assistance Force (ISAF) service members died when two individuals wearing Afghan National Security Forces' uniforms shot them in eastern Afghanistan," the NATO force said in a statement about Wednesday's attack.

"ISAF and Afghan officials are assessing the incident to determine the facts."

A Pentagon official in Washington, speaking on condition of anonymity, later told AFP the two soldiers killed were Americans.

The incident happened in the province of Kapisa, north of Kabul, provincial governor Mehrabudin Safi said, giving no further details.

Taliban spokesman Zabiullah Mujahid claimed the militants were responsible for the killings.

But ISAF officials say that most insider attacks stem from personal grudges and cultural misunderstandings rather than Taliban insurgent plots.

The number of such attacks fell sharply in 2013, but the threat still means that foreign soldiers working with Afghan forces are regularly watched over by so-called "guardian angel" troops to provide protection.

Afghan soldiers and police are taking on responsibility for battling the Taliban from about 55,000 NATO combat troops who will leave by the end of 2014 -- 13 years after a US-led invasion brought down the Taliban regime.