By Jack Kimball and Sanjeev Miglani
KABUL (Reuters) - Three foreign soldiers, including two Britons, were shot dead by Afghan security forces personnel on Monday in the latest round of so-called insider killings which have raised deep concerns about the reliability of NATO's local allies and their ability to keep the peace.
Insider attacks have mounted as tension between Afghanistan and its foreign backers rises over a series of incidents, including the burning of Korans at a NATO base and a massacre of 17 villagers for which a U.S. soldier has been charged.
U.S. Defence Secretary Leon Panetta said despite a recent spate of killings of NATO troops by Afghan soldiers, such violence was sporadic and did not "reflect any kind of broad pattern."
"We have to keep our eye on the fundamental mission here, which is to accomplish our main strategy of being able to make the transition to Afghan control and security," he told reporters en route to a defence ministers meeting in Canada.
In Monday's attacks, an Afghan army soldier killed two British soldiers at their headquarters in southern Afghanistan, Britain and NATO officials said, while a local policeman shot dead another foreign soldier in the east.
The attacks on foreign troops by Afghan security personnel have raised doubts over local forces' readiness to take over security responsibilities by the end of 2014, when most Western combat troops are to leave.
The attack on British troops took place in Lashkar Gah city in southern Helmand province, the main area of operations for British forces in Afghanistan.
"It appears that a member of the Afghan national army opened fire at the entrance gate to the British headquarters at Lashkar Gah city, killing the two British service personnel," Britain's defence secretary, Philip Hammond, told parliament in London.
The attacker was shot dead by NATO soldiers, the alliance and the governor's office said.
Later on Monday, NATO said that a member of the Afghan local police shot and killed one of its soldiers near a checkpoint in eastern Afghanistan.
"The circumstances behind the incident are currently under investigation by Afghan and coalition forces," the International Security Assistance Force said. The local police force has been set up to defend villages in areas where the presence of national forces is weak.
PREVENTING ATTACKS 'NOT EASY'
Panetta said General John Allen, commander of NATO and U.S. forces in Afghanistan, was putting in place steps to try to prevent such incidents.
"This is not easy," he said. "There are going to be those that are vengeful, there are going to be those that decide to use this as a way to express their anger and their concern."
No group has claimed responsibility for either attack. The Helmand provincial governor's office said the base where the British soldiers were attacked housed military and civilian reconstruction teams.
The latest deaths in Helmand bring to 407 the number of British soldiers killed in Afghanistan.
Before Monday's attack, 13 members of the NATO-led force had been killed this year in what appeared to be attacks by members of Afghan forces, Allen told a U.S. Senate committee last week.
About 70 members of the NATO force have been killed in 42 insider attacks from May 2007 to January this year.
Some incidents have been carried out by Afghan security forces reacting to the Koran burning last month, some have been due to private grievances and others have been carried out by Taliban insurgents who infiltrated the security forces.
An Afghan army general has said that the Taliban have a sophisticated system to infiltrate Afghanistan's security forces and vetting of recruits must be severely tightened.
The Taliban have proven resilient in the face of far superior Western firepower. But poor management of the recruiting process for the army and police has also given them an opportunity to infiltrate.
(Additional reporting by Hamid Shalizi in Kabul, Mohammed Abbas in London and David Alexander in Ottawa; writing by Sanjeev Miglani; editing by Giles Elgood and Mohammad Zargham)