A Royal Marine and a soldier have died from injuries sustained while on patrol in Afghanistan, the Ministry of Defence said.
The Royal Marine, from 40 Commando and the soldier, from 3 Medical Regiment, were on patrol in the Nahr-e Saraj district of Helmand province. Their families have been informed.
Spokesman for Task Force Helmand, Major Laurence Roche, said: "I am extremely sorry to announce the deaths of a Royal Marine from 40 Commando and a soldier from 3 Medical Regiment serving with Task Force Helmand. This is dreadful news for all of us serving in Afghanistan. Our sincere condolences go to their families, friends and colleagues at this time of grief."
Speaking from Afghanistan, BBC's defence correspondent Caroline Wyatt said according to one Afghan source, the deaths were from a "green on blue" attack - where coalition troops are killed by their Afghan allies - but other local sources disputed that.
A source said it understood the Taliban had issued a statement in Afghanistan suggesting the incident was a green on blue attack.
But the source said the details of the incident were still unclear because the people who knew what happened were not in a "state" to write down witness statements or explain what they saw.
The source said the Marine and soldier appeared to have been killed by gunshot wounds by insurgents, but the details had not yet been confirmed.
Their deaths take the total number of UK service members to have died since operations in Afghanistan began in October 2001 to 435.