The "dreadful" murder of three British soldiers by a member of the Afghan security forces will not succeed in undermining the military cooperation between the two countries, dpa quoted Prime Minister David Cameron as saying Tuesday. It was "absolutely essential" that the strategy of working alongside the Afghan National Army (ANA) continued, stressed Cameron, adding that trust between the two forces was "strong and working well." "We need to make sure we build on that army and that in the end is the way that we are going to be able to bring out troops back home," said Cameron. He said the attack was likely to have been carried out by "rogue elements" in the Afghan National Army. The incident, in which a further four British soldiers were injured, was condemned as a "despicable and cowardly act" by Defence Secretary Liam Fox. It brings to 317 the number of British soldiers killed in Afghanistan since the conflict began in 2001. The defence ministry said the three men who died Tuesday were from 1st Battalion Royal Gurkha Rifles. The Afghan authorities said the attacker had escaped capture. "An Afghan army soldier opened fire on the British soldiers in Babaji area of Helmand and killed three British forces and injured four others," General Zahir Azimi, spokesman for the Defence Ministry, said. The NATO-led International Security Assistance Force (ISAF) confirmed three of its soldiers had been killed and several others injured in an attack. "A joint ISAF and Afghan Ministry of Defense team is investigating the incident due to allegations the ISAF service members were fired upon by an Afghan National Army soldier," a statement said. Wahid Omar, the chief spokesman for President Hamid Karzai, said the president was "upset" to hear about the attack. "We believe these were the actions of a lone individual who has betrayed his ISAF and Afghan comrades," the ministry said in a statement, adding: "His whereabouts are currently unknown but we are making strenuous efforts to find him." "I echo the condolences and sentiments offered by President Karzai and the other Afghan officials," General David Petraeus, the top US and NATO commander in Afghanistan, said in a statement. "This is a combined, joint mission, Afghan and Alliance troopers fighting shoulder-to-shoulder against the Taliban and other extremists," he said. "We have sacrificed greatly together, and we must ensure that the trust between our forces remains solid in order to defeat our common enemies." Afghanistan's Chief of Army Staff Sher Mohammad Karimi expressed his condolences and vowed prosecution of those responsible. There are more than 9,000 British soldiers based in the province. The British troops are part of 140,000 US and NATO personnel currently stationed in Afghanistan. Besides battling the resurgent Taliban fighters, the NATO troops are also tasked with training the Afghan security forces. The combined forces also work with Afghan troops in their operations. Five British soldiers were killed by an Afghan policeman in the same Helmand province in November. In London, Cameron and Fox stressed Tuesday that the training of Afghan National Security Forces was "vital" to the international mission in Afghanistan. -- SPA