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

A spokesman for NATO's International Security Assistance Force (ISAF) said the attackers were believed to be insurgents 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.

If it is confirmed as a green-on-blue attack, in which Afghan forces turn their weapons against their Western allies, it would take the toll this year to 22 NATO soldiers killed in at least 16 such attacks.

Helmand province police spokesman Farid Ahmad Farhang told AFP 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.

A senior security official in the province, Mohammad Ismail Hotak, 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 who are helping Kabul fight a decade-long insurgency by hardline Taliban Islamists.

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.

Aside from the "green-on-blue" shootings, relations between the United States and its Afghan allies have been rocked this year by a series of incidents involving American troops.

Videos and pictures have emerged of US forces abusing Taliban corpses, copies of the Koran were burnt on a major US military base and an American sergeant has been charged with 17 counts of murder over a massacre of civilians.

And on Friday, NATO and US forces admitted that a number of civilians were killed in two separate airstrikes, days after President Hamid Karzai warned that such deaths threatened relations with the US.

A joint statement by ISAF and US forces gave no details of how many civilians died in each incident, but local officials have put the total at more than 20, including women and children.

Karzai had summoned ISAF commander John Allen and US ambassador Ryan Crocker to warn that civilian deaths caused by international forces threatened the strategic pact he signed with US President Barack Obama earlier this month.

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

Two other NATO soldiers died Saturday, one in a bomb blast and the other as a result of a "non-battle related injury", ISAF said, without identifying the nationalities of any of the victims.

Soldiers shot by Afghan police were British: London
London (AFP) May 13, 2012 - Two soldiers shot dead in southern Afghanistan by local police officers were British, the Ministry of Defence confirmed on Sunday a day after the killings.

"Sadly, it is my duty to confirm that a soldier serving with 1st Battalion Welsh Guards and an airman from The Royal Air Force have been shot and killed in the Lashkar Gah district of Helmand province," said Major Ian Lawrence, spokesman for Task Force Helmand.

The ministry said in a statement that the two were "shot and killed by members of the Afghan police force", although NATO's force in Afghanistan had on Saturday indicated they were insurgents dressed as police.

A police spokesman said they were members of the force, however, and a senior security official in the province said they had been in the Afghan police force for about a year.

The two had been working as part of an advisory team and were providing security for a meeting with local officials at the time, the ministry's statement said.

ISAF said Saturday that one of the attackers had been killed and the other was still being sought.

The killings bring this year's toll in "green-on-blue" attacks -- in which Afghan forces turn their weapons against their Western Allies -- to 22, in a total of 16 such incidents.

An increasing number of Afghan troops have turned their weapons on NATO soldiers who are helping Kabul fight a decade-long insurgency by hardline Taliban Islamists.

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.

The families of the two British soldiers have been informed, the ministry said.

The killings bring the British toll in the more than 10-year Afghan war to 412.

Britain has some 9,500 troops in Afghanistan, mainly based in Helmand.

They form part of a 130,000-strong ISAF force fighting alongside some 350,000 Afghan security personnel against the Taliban-led insurgency, but the foreign troops are due to pull out of the country in 2014.

Two other NATO soldiers died Saturday, one in a bomb blast and the other as a result of a "non-battle related injury", ISAF said, without giving the nationalities of any of the victims.