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, ISAF, had on Saturday indicated the killers were insurgents dressed as police.
A police spokesman said they were members of the force, however, and an Afghan security official said they had been in the police force for about a year.
ISAF said Saturday that one of the attackers had been killed and the other was still being sought.
The two British soldiers were part of an advisory team and were providing security for a meeting with local officials at the time, the ministry's statement said.
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.
Britain's defence minister Philip Hammond told BBC television on Sunday that it was "still very early days" in terms of information about the attack, and that the killers' motives were unclear.
They may have been insurgent infiltrators or officers with individual grievances, he said.
"British forces work alongside Afghan forces every day, with thousands and thousands of contacts with them every day," he added.
"This is a country that has an insurgency going on in it, and sadly sometimes these events occur.
"Remember that this is a society where people traditionally settle grievances by violence."
An increasing number of Afghan troops have turned their weapons on NATO soldiers 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 Afghan security forces, but many are attributed to cultural differences and antagonism between the allied forces.
The families of the two British troops 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, 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.