There are more than 130,000 foreign troops battling Al-Qaeda and Taliban militants in Afghanistan
An Afghan police officer has killed three British soldiers serving with NATO in Afghanistan's troubled south, the latest in a series of escalating "green on blue" attacks in the decade-long war.
The deaths on Sunday take to at least 26 the toll so far this year from 18 attacks in which Afghan forces turned their weapons against their Western allies.
The defence ministry in London confirmed the soldiers were British, serving with an Afghan police advisory team, and were killed after meeting local elders in the Nahr-e-Saraj district of Helmand province.
In keeping with policy, NATO's International Security Assistance Force (ISAF) gave few details of the incident, which happened around 5:00 pm (1230 GMT) on Sunday, but said the gunman was wounded and detained after the attack.
"An individual wearing an Afghan National Civil Order Police uniform turned his weapon against International Security Assistance Force service members in southern Afghanistan... killing three service members," ISAF said.
Helmand provincial spokesman Daud Ahmadi confirmed the man who opened fire was a member of the civil order police, an elite riot control force set up in 2006.
An increasing number of Afghan troops have turned their weapons against NATO colleagues helping them fight a decade-long insurgency by hardline Taliban Islamists.
The latest attack comes less than two weeks after three men in Afghan police uniforms killed a soldier with the US-led force, also in the south.
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.
Prime Minister David Cameron said he was "deeply saddened" by the killings, while Defence Minister Phillip Hammond vowed that the "cowardly" attack would not deter British troops from their mission to build up Afghan forces.
"Every day, tens of thousands of coalition forces, including UK personnel, live and work successfully with their Afghan counterparts to build an Afghan police force and army which can take the lead for their own security by the end of 2014," Hammond said.
"That process will continue, and though deeply tragic, yesterday's incident and attacks like it will not derail the mission or distract us from the task in hand."
NATO has around 130,000 soldiers fighting alongside some 350,000 Afghan security personnel, but Western combat troops are due to pull out of the country in 2014.
The coalition is to hand over security to local forces by mid-2013 and will play a support role up to the final withdrawal by the end of the following year.