Two NATO troops have been killed by two men in Afghan police uniforms in southern Afghanistan, the military said, with local sources identifying the victims as British soldiers based in Helmand province.

A spokesman for NATO's International Security Assistance Force (ISAF) said on Saturday the attackers were believed to have been dressed as police, but a senior Afghan security official said they had been in the police force for a year.

"We are aware of the claims that the shooters were AUP (Afghan Uniformed Police); however, operational reports indicate these were insurgents dressed in AUP uniforms and not actual AUP," ISAF said.

"The event is still being assessed and additional information may be released as appropriate," a spokesman said.

Helmand province police spokesman Farid Ahmad Farhang told AFP news agency that "two Afghan policemen opened fire on British soldiers inside a joint military base in Nahre Saraj district".

One of the police shooters "was killed when foreign troops returned fire, the second policeman has been injured", he said.

'Guardian angels'

Mohammad Ismail Hotak, a senior security official in the province, confirmed the attack, saying the policemen had been in the force for a year.

ISAF confirmed that one of the attackers was killed, adding that the other "is being sought".

An increasing number of Afghan troops have turned their weapons against NATO soldiers, with at least 20 international troops killed in attacks this year.

Some of the assaults are claimed by the Taliban, who say they have infiltrated the ranks of Afghan security forces, but many are attributed to cultural differences and antagonism between the allied forces.

ISAF has taken several security measures in response to the shootings, including assigning "guardian angels" - soldiers who watch over their comrades as they sleep.

ISAF has around 130,000 soldiers fighting alongside some 350,000 Afghan security personnel, but they are due to pull out of the country in 2014.