A British soldier has been shot dead in Afghanistan by a man in Afghan army uniform, the latest in a series of such attacks.
More than 60 troops from the force were killed in 2012 in a surge of insider attacks that have shaken relations between the alliance and Afghanistan.
Taliban militants often claim such insider attacks are planned strikes by its supporters, but Isaf and Afghan officials say that many are due to personal grudges.
The latest victim, a soldier from 28 Engineer Regiment, attached to 21 Engineer Regiment, was killed on Monday in Afghanistan.
The gunman was also killed during the attack, which took place in Patrol Base Hazrat, in the Nahr-e Saraj district of Helmand Province.
The male soldier's next of kin have been informed.
Coincides with Karzai visit
The attack comes as President Karzai is due in the US for a visit seen as key to the future of the American presence in Afghanistan.
Spokesman for Task Force Helmand, Major Laurence Roche, said: "It is my painful duty to report the death of a soldier from 28 Engineer Regiment, who was shot and killed by a suspected member of the Afghan National Army at a patrol base in the Nahr-e Saraj district of Helmand Province.
"This is an extremely sad day for the Corps of Royal Engineers and everyone serving with Task Force Helmand. Our thoughts are with the soldier's family and friends at this time."
The BBC reported the gunman had first opened fire on Afghan National Army troops, then British soldiers, killing the British soldier and injuring a number of other people. The gunman was killed in a return of fire.
He said the Taliban have claimed the gunman was their infiltrator.
An investigation is under way to determine if the attacker had links to insurgents, the International Security Assistance Force (Isaf) said in a statement.
There has been a sharp rise in so-called insider attacks against local forces and Nato troops in Afghanistan. More than 60 Nato service personnel and a quarter of the British troops who died in Helmand in 2012 were killed in such attacks.
Military commanders refer to such rogue attacks as "green-on-blue" - green for Afghan forces and blue for the Nato coalition.
President Hamid Karzai is expected to arrive in the US for a three-day visit, which will include a meeting with President Barack Obama on Friday.
It will be their first face-to-face talks since the US presidential elections.
Correspondents say relations between the two countries have been more fractious than usual in recent months.
Mr Karzai wants US troops out of Afghan towns and villages. Some 30,000 foreign soldiers left Afghanistan last year, and most of the remainder are expected to pull out by the end of 2014.
