article: The men, from 1st Battalion The Royal Gurkha Rifles, were on duty on Tuesday morning in Nahr-e Saraj, Helmand province, when the Afghan opened fire. One is believed to be a Nepalese Gurkha and the other two were UK nationals. Prime Minister David Cameron condemned the "appalling" killings, but said UK and Afghan troops should work together. The Taliban has claimed the Afghan soldier had joined its insurgency. An investigation is under way and next of kin have been informed. Meanwhile, in a separate incident a Royal Marine from 40 Commando was killed in Helmand, the MoD said. Family have been told. He was shot dead on Tuesday while on foot patrol in the Sangin district. The number of British military personnel killed on operations in Afghanistan since 2001 now stands at 318. Mr Cameron said the current strategy of working alongside local security forces should go on. "It is absolutely essential that we don't let this terrible incident change our strategy. It is the right thing to do to build up the Afghan national army," he said. He went on: "We need to make sure that we build up that army because that, in the end, is the way that we are going to be able to bring our troops back home." Similarly, Defence Secretary Liam Fox said: "Training and developing the Afghan National Security Forces is vital to the international mission in Afghanistan and today's events will not undermine the real progress we continue to make. British and Isaf forces are working shoulder to shoulder with Afghans and will continue to do so undeterred." Afghan President Hamid Karzai has apologised to the UK after the incident and a defence ministry spokesman said a soldier from the Afghan National Army (ANA) was being sought. By Jonathan BealeDefence correspondent, BBC News This is not the first time this kind of incident has happened. Last November a member of the Afghan National Police opened fire on British soldiers he was serving alongside. He killed five and seriously injured half a dozen others. But the Ministry of Defence is anxious to highlight how rare these kinds of incidents are. Since 2006, British forces have trained 130,000 Afghan troops. Some 5,000 of the 10,000 British servicemen currently deployed in Afghanistan are working alongside members of the Afghan Security forces day in and day out. The training and mentoring of Afghanistan's army and police is essential to the goal of eventually leaving an Afghanistan capable of looking after its own security. That is why Liam Fox is stressing that, despite this tragedy, British forces will "continue to work shoulder to shoulder with the Afghans" and will not be deterred. Royal Marine dies in Afghanistan Mr Cameron said he spoke to President Karzai on Tuesday morning and both men agreed that an urgent investigation was required. President Karzai's spokesman, Waheed Omar, said the Afghanistan government was also investigating the killings and the government would "do everything to ensure the perpetrators are brought to justice". The BBC understands that one of the soldiers killed was shot and the
summary: Three British soldiers have been killed and four injured by a renegade Afghan soldier in Afghanistan, the Ministry of Defence said.