He told BBC One's Andrew Marr show so-called green-on-blue attacks - in which members of the Afghan security forces attack international allies - were rare, and the motivation for the latest incident remained unclear
There have been 22 "green-on-blue" deaths - mostly Americans - so far this year, compared with 35 for the whole of 2011.
A dozen British service personnel have been killed in such attacks since 2009.
Afghan intelligence officials have told the BBC the Taliban want to create a climate of mistrust where Afghan and Nato soldiers cannot work together.
The BBC's Bilal Sarwary said a senior Afghan official in Kabul said the issue of rogue soldiers and Taliban infiltration was an even bigger threat than suicide attacks and Taliban attacks.
He said it was a Taliban "strategy, not a tactic" applied by insurgent groups across Afghanistan, which had officials worried despite their insistence that fresh security measures were in place.
Around 130,000 coalition troops are fighting alongside 350,000 Afghan security personnel against the Taliban-led insurgency.
International combat troops have begun their withdrawal and are due to pull out of Afghanistan by the end of 2014.
Key to that timetable is the training and mentoring of Afghan forces so they can take control of the security of their country themselves.
Shadow defence secretary Jim Murphy told Sky News more needs to be done to protect British troops who remain in Afghanistan.
He said: "If these killers, who carried out this attack, weren't even members of the armed forces then it raises another question which is that in 2015 - when Britain is only doing a training role in Afghanistan and our combat troops have been fully withdrawn - who's going to look after the British trainees."
Source is
BBC News related article
Guests: 74
Hidden: 0
Users: 0