A British soldier has been shot dead and six others injured by a rogue member of the Afghan National Army.
The UK soldier, of 28 Engineer Regiment, attached to 21 Engineer Regiment, died at Patrol Base Hazrat, in Helmand's Nahr-e Saraj district.
The gunman fired at Afghan troops and then British soldiers before being killed. Afghan officials dispute Taliban claims he was acting for them.
The male soldier's next of kin have been informed of Monday's attack.
Prime Minister David Cameron's spokesman described it as a "tragic incident", adding: "The PM's thoughts are very much with the family and friends of the soldier who was killed."
The spokesman said the military had stepped up counter-intelligence efforts and the vetting and screening of new Afghan National Army recruits.
The attack comes as President Hamid 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: "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."
Rise in attacks
The BBC's Quentin Sommerville said the attack is thought to have been triggered by an argument between Afghan National Army soldiers.
Shortly after details of the incident emerged, the Taliban said they were behind the shooting, but they often wrongly claim such attacks, he said.
Afghan defence officials told the BBC that initial assessment suggested the gunman was a soldier from the eastern province of Laghman.
An official said: "Currently there is an investigation into the attack. Initial assessment and information shows the attacker was not linked to the Taliban.
"After he killed the British soldier, he tried to escape but members of the Afghan national security forces opened fire, killing him.''
There has been a sharp rise in so-called insider attacks against local forces and Nato troops in Afghanistan. In 2012, more than 60 Nato service personnel, and a quarter of the British troops who died in Helmand, were killed in such attacks.
All six of the British troops killed during the latest six-month tour of duty have died this way. A total of 439 UK service members have lost their lives in Afghanistan since operations began there in October 2001.
President Karzai is arriving 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.
Our correspondent says the US president is likely to discuss the size of the force that remains, and whether they are there to fight the Taliban or shift their primary focus to tackling al-Qaeda.
Mr Karzai's visit comes days after a drone attack that killed Pakistan militant leader Mullah Nazir, who was accused of sending fighters to Afghanistan to support the Taliban.
Ahead of the trip, the White House issued a statement saying Mr Obama "looks forward to... discussing our continued transition in Afghanistan, and our shared vision of an enduring partnership" between the two countries.
Mr Karzai is expected to provide a request for future requirements for the Afghan military such as heavy weaponry, an improved air force and medical support.
Our correspondent says the US spent almost $120bn (£74bn; 91bn euros) in Afghanistan in 2011, a figure that diplomats in Kabul say will be increasingly difficult to justify because of economic problems at home and extensive corruption within Afghanistan.
Meanwhile Mr Karzai has held back from signing a long-term binding security agreement with the US, amid fears that Afghan sovereignty is being undermined.
He is unwilling to grant US soldiers immunity from prosecution and is unhappy that Afghans are still being held in US-run military prisons on Afghan soil, our correspondent adds.
Nato forces are committed to training Afghan security forces before foreign combat troops pull out.