A man wearing Afghan army uniform shot dead two soldiers from NATO's US-led force in southern Afghanistan on Monday, a military spokesman said.

The deaths come after a series of "green-on-blue" attacks involving Afghan troops targeting their international allies, who are scheduled to hand over responsibility for national security to the Afghans by the end of 2014.

"An individual wearing Afghan army uniform turned his weapon against International Security Assistance Force service members in southern Afghanistan today, killing two," ISAF spokesman Major Jason Waggoner told AFP.

Coalition forces returned fire and killed the gunman, he added, declining to specify the victims' nationality.

A senior Afghan official told AFP the incident happened inside a US-run base used by civilian and military personnel working in a provincial reconstruction team in Lashkar Gah, the main town in Helmand province.

An ISAF statement released in Kabul said a joint Afghan and NATO investigation had been opened into the killings.