Two British troops have been shot dead by an Afghan Army soldier in Lashkar Gar.
The Royal Marine and the Adjutant General's Corps soldier were serving as part of Task Force Helmand when the gunman opened fire at the gate of the British headquarters.
Afghan military leaders say it is unclear whether the man was acting alone or had been "influenced by the enemy".
Lieutenant Commander Brian Badura from Isaf said the gunman had been shot dead and an investigation will be carried out into the attack.
The attack appears to be the latest in a string of "green on blue" attacks in which members of the Afghan security forces have opened fire on their international allies.
Five British soldiers were killed at a military compound in Nad e-Ali in November 2009, when a rogue policeman opened fire on them. The Taliban later claimed responsibility for the attack.
Defence Secretary Philip Hammond said: "Our thoughts, as ever, are with their families, for whom this will be a deeply personal tragedy."
Isaf has revealed that a third Nato solidier was also killed by Afghan security forces in a separate incident.
The servicemen, who is believed to be American, was shot dead by an Afghan policeman at a checkpoint in the east of the country.