An Afghan policeman shot dead two soldiers from the US-led NATO coalition, the latest incident of a gunman in uniform targeting foreign personnel, NATO officials said Friday.

The policeman opened fire as the soldiers, whose nationalities have not been released, prepared to eat lunch at a police base in the southern province of Helmand, said an International Security Assistance Force (ISAF) release.

It said the victims of Thursday's attack were part of a mentoring team, one of many working to train up the Afghan police and army to take over security across the country in the next three to four years.

"The shooter was seriously injured during the exchange and is currently in a medical treatment facility," ISAF said in the release, adding that an investigation had been launched.

Yousuf Ahmadi, a Taliban spokesman, said in a website statement that the policeman was an undercover Taliban agent who joined the force to carry out the attack and that 13 people had been killed.

Rebel spokesmen in Afghanistan regularly exaggerate death tolls and make unproven claims of responsibility.

NATO's efforts to train local forces to take responsibility by the end of 2014 have been hit by a string of attacks by militants who have apparently infiltrated units or who attacked while wearing unauthorised uniforms.

Last month an Afghan soldier blew himself up near the eastern town of Jalalabad, killing five foreign troops and four Afghans.

On April 27, a former Afghan pilot shot dead eight US troops and an American contractor at a Kabul training centre in a reported argument, in the worst such incident since the 2001 US-led invasion brought down the Taliban regime.

In November, six US troops were shot dead during a training session in eastern Afghanistan, apparently by a member of the Afghan border police.

The motivation behind Afghan security personnel turning on coalition troops is often unclear, but each incident has raised questions over ISAF's crucial efforts to expand and train the army and police.

Major General James Laster, an ISAF spokesman, said that Thursday's attack was "a serious incident (but) the actions of this individual do not reflect the overall actions of our Afghan partners."

"We remain committed to our partners and to our mission here," he said.

There are around 130,000 international troops serving in Afghanistan, the bulk of them from the United States, although Afghan forces are in control of security in Kabul.