Afghan policeman kills three British soldiers KABUL/LONDON (Reuters) - An Afghan policeman shot dead three British soldiers at a checkpoint in southern Helmand province on Sunday, Afghan officials said, the latest in a chain of increasingly frequent rogue killings. A fourth British soldier was also injured, provincial governor spokesman Daoud Ahmadi said of the attack, which could further erode trust between NATO and the Afghan forces they train before most foreign combat troops leave in 2014. The soldiers were serving with an Afghan police advisory team and were killed after a meeting at the checkpoint at Nahr-e-Saraj in Helmand, Britain's Defence Ministry said on Monday. Defence Secretary Philip Hammond said the deaths, which take to 422 the number of British military personnel killed in Afghanistan since the NATO-led war began in 2001, would not impact Britain's mission there. "Though deeply tragic, yesterday's incident and attacks like it will not derail the mission or distract us from the task in hand," Hammond said in a statement. So-called green on blue shootings, referring to the uniform of the Afghan security forces and the color of NATO, have severely strained ties between President Hamid Karzai and his Western backers. Before Sunday's assault there were 17 such attacks this year and 23 NATO soldier deaths, compared to the 11 attacks and 24 deaths for the same period last year, NATO figures show. But NATO commanders argue the growing number of shootings is in proportion to the growing size of Afghan security forces toward an eventual 352,000 target. After the United States, Britain has the second-largest number of troops in Afghanistan at about 9,500, and is due to withdraw 500 by the end of the year. Reporting by Tim Castle in London, Mirwais Harooni and Amie Ferris-Rotman in Kabul; Editing by Ed Lane