Two British servicemen have been shot dead by members of the Afghan Police Force.
A Royal Air Force airman and a soldier from 1st Battalion Welsh Guards were killed at a patrol base in the Lashkar Gah district of Helmand province, the Ministry of Defence said.
The servicemen were providing security for a meeting with local officials when two people wearing Afghan Police uniforms opened fire.
Defence Secretary Philip Hammond said it appeared that two assailants were involved in the attack.
"What appears to have happened is that an Afghan police officer opened fire on a mentoring team working with the Afghan police. One of the assailants was killed, we think, by other Afghan police officers, one escaped," he told BBC1's The Andrew Marr Show.
He added: "British forces work alongside Afghan forces every day with thousands of contacts with them every day. This is a country that has an insurgency going on in it and, sadly, occasionally, these events occur.
"We don't yet know what the motive was, we don't yet know whether this was an insurgent who'd infiltrated the police or whether it was a policeman who simply had a grievance of some kind.
"This is a society where people traditionally settle grievances by violence."
Major Ian Lawrence, spokesman for Task Force Helmand, said: "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.
"The thoughts and condolences of everyone serving in the Task Force are with their families and friends."