Two British soldiers have been killed in the big British barracks at Lashkar Gah, Helmand province, by an Afghan wearing a military uniform.
An American soldier was killed in a separate attack in eastern Afghanistan by a man believed to be part of a village-level fighting force being fostered by the Americans.
One-in-seven international military fatalities in Afghanistan this year has been caused by Afghan soldiers turning their arms on the men who thought they were comrades.
Author and former SAS officer Andy McNab has just returned from Helmand and told the Today programme's Sarah Montague that "even the Afghan National Army (ANA) are targets," pointing out that the brigade commander does not go anywhere without his bodyguard.
He outlined the three ways the ANA could be attacked: Infiltration, intimidation and grievance.
"They have been fighting a war against the Taliban as well" he said.
He said that if there is a disagreement, "they sort it out the Afghan way" but insisted that the "relationship between the ANA and the British advisory brigades is very good" adding that they were in fact 18 months ahead of everyone else.
The ANA are very aware of the killings in a village by an American soldier recently but they can differentiate.
Mr McNab was positive about the future of withdrawal saying "a lot of commentators are out of date".
"The transition has been so successful and so fast on a military level," he said.
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