There is mounting concern over a sharp rise in attacks on Western troops by members of Afghanistan's security forces.

So far this year more than 30 soldiers have been killed in the so-called "green on blue" killings.

The past weekend was one of the deadliest periods for coalition troops in Afghanistan this year, with six US soldiers killed in separate attacks in southern Afghanistan's Helmand province.

Early on Friday three US Marines were gunned down by a Afghan police commander and his men after accepting an invitation to have dinner.

Later the same day an Afghan civilian working at a NATO base shot dead three more US soldiers.

And on Saturday an Afghan police officer described as an "Taliban infiltrator" shot dead at least 10 of his colleagues in the south-western province of Nimroz.

US officials have condemned the attacks but White House spokesman Jay Carney says it is not clear if they are part of a wider pattern.

"It is too early to say that this latest incident is part of a stepped up effort on the insurgents' part," he said.

Brigadier-General Gunter Katz is the chief spokesman for the International Security Assistance Force based in Kabul, and he says private disputes are being put forward as one reason for the attacks.

"Most of those incidents were either caused by personal grievances or by stress situations of the individual who did the shooting," he said.

Sapping morale

But the figures are hard to ignore. Dozens of Western soldiers have died at the hands of their Afghan allies, including four Australians.

This year 34 soldiers have died in green on blue killings - more than in all of last year.

The situation is sapping morale and eroding trust between allies ahead of the coalition withdrawal in 2014.

Joshua Foust is an Afghanistan expert with the American Security Project.

He says green on blue killings are being deliberately employed by the Taliban and are proving to be very effective for a number of reasons.

"Soldiers who work in the training mission have reported that there's a lot of condescension between American troops and Afghan troops, or a lot of stress built into that relationship, just because of the nature of the war, and that that's been having a toll on Afghan soldiers," he said.

"But also when you talk with people who've participated in previous years on the training mission itself, they complain of things like essentially grade inflation, of rating units to be actually more functional than they actually are.

"Recent surveys that have come out have also shown that ordinary soldiers, not trainers, but ordinary soldiers in the US Army don't have very high confidence in the Afghan Army itself either.

"So there's a big difference between measurable progress and the ability of these Afghan units to really take on the role of creating security."

Mr Foust says it is another indication of how the Taliban is becoming increasingly sophisticated in its methods and strategy.

"The last several years have seen this rise of they call them spectacular attacks against government targets and US targets inside Kabul, and they've been doing that with the purpose of trying to pierce this ring of security that's been built around the capital and around all the Western powers there.

"By attacking those targets what the Taliban have done is undermine confidence in both the presence of international forces and then also in the Afghan government.

"That's the kind of thing that you end up doing in a late stage of an insurgency when your goal is to de-legitimise the government that's in power right now, and not necessarily to rack up a high body count."