British troops killed two of their own soldiers in a "friendly fire" incident in Afghanistan while two American soldiers were shot dead by an Afghan ally, Afghan and international military sources said Thursday.

Britain's defence ministry did not confirm the deaths of the two British soldiers in Helmand province -- one of them a female medic -- were a result of "friendly fire", saying the incident was still under investigation.

But US forces in Afghanistan confirmed that two American soldiers were killed by a man in an Afghan police uniform -- the latest in a series of insider attacks that have seriously undermined trust between the allies.

Helmand police spokesman Farid Ahmad Farhang said the British deaths were due to a mistaken belief by a British patrol that they were under fire from insurgents -- while the shooting actually came from a second British patrol.

"There were two groups of British soldiers foot-patrolling an area called Malgir in Greshk district yesterday at around 5:00pm (1230 GMT)," Farhang said.

"As one group proceeded to a village they saw a policeman in civilian clothes performing ablutions (before prayers). The British soldiers thought he was a Talib and opened fire on him, killing him on the spot.

"The second group of British soldiers who were coming from a distance thought they were attacked by the Taliban and opened fire in the direction they had heard the gunfire, killing two of their colleagues," Farhang said.

This version of events was confirmed by Mohammad Ismail Hotak, the head of the coordination centre of Afghan forces in Helmand province.

Farhang said local officials with NATO's International Security Assistance Force (ISAF) had also "confirmed it was a mistake and friendly fire which killed the two British".

In Kabul, however, an ISAF spokesman told AFP: "We are tracking that reporting and at this point I'll call it a rumour that's out there.

"Unfortunately there is an ongoing assessment being done to look into the cause of that event and we do not have definitive operational reporting at this time to confirm that.

"Hopefully we will get some kind of resolution sometime today or tomorrow."

Britain's ministry of defence said: "There was an exchange of gunfire that resulted in the deaths of a Royal Marine... a female soldier from 3 Medical Regiment and an Afghan man who is believed to be a member of the Afghan Uniformed Police but who was not wearing uniform at the time.

"The UK patrol were not working with any Afghan partners at the time."

The spokesman added: "At this stage we do not know what initiated the exchange of gunfire and an investigation is ongoing. Further details will be provided as information becomes available but at this time the situation remains unclear."

The MoD later named the dead soldiers as Corporal David O'Connor of 40 Commando and Channing Day of 3 Medical Regiment.

The Afghan conflict has seen an alarming surge in so-called "green on blue" insider attacks this year, with more than 50 soldiers from NATO's US-led International Assistance Force (ISAF) killed by their colleagues in the Afghan army and police.

The attacks have created deep distrust between the allies, and have rocked NATO'S plan to train Afghan forces to take over the fight against the Taliban when foreign troops leave.

The unprecedented number of insider killings comes at a critical moment in the 11-year war, as NATO forces train Afghans to take over security responsibility before the withdrawal of foreign combat forces by the end of 2014.

Thursday's attack on the US soldiers happened in the central province of Uruzgan.

"Two US Forces-Afghanistan service members died after an individual wearing an Afghan National Police uniform turned his weapon against them in Khas Uruzgan," the US military in Afghanistan said, referring to a district in the province.

It is believed to be the first such attack since September 30, when a firefight between NATO troops and their Afghan allies killed five people, including one US soldier and a civilian contractor.

Most insider attacks are blamed on cultural differences and personal grievances between soldiers working closely together against Taliban insurgents, but NATO attributes some 25 percent of the incidents to Taliban infiltrators.

Elsewhere on Thursday, Italy's defence ministry said an Italian soldier had died after being wounded in a shooting at Bakwa in Farah province.

The ministry said 24-year-old Tiziano Chierotti had succumbed to injuries sustained in the attack which injured three other soldiers.

The death brought to 52 the number of Italian soldiers to have died in the 11-year Afghan conflict, according to an AFP count.

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