Seven international soldiers have been killed by their local colleagues in a bloody week of violence in Afghanistan, further eroding trust between foreign troops and the Afghans they work with.

Six died on Friday alone -- three American soldiers were shot by an Afghan policeman who invited them to a meal and three other troops were killed by an Afghan civilian employed on a NATO base, military and Afghan officials said.

NATO has about 130,000 soldiers helping the Afghan government fight an insurgency by Taliban Islamists, but they are due to pull out in 2014 and are increasingly working with Afghans they are training to take over.

So-called green-on-blue attacks, in which Afghans turn their weapons against their foreign allies, have killed a total of 34 international soldiers this year, according to a NATO count.

The seven deaths this week -- one soldier was killed on Tuesday -- make up around 40 percent of the total of some 17 foreign troops killed.

"Clearly as far as the future partnering and training and mentoring of Afghan forces by NATO and the US is concerned, it is going to have a very negative effect and the lack of trust between the two sides is going to grow," said author and analyst Ahmed Rashid.

"NATO will have to impose new security measures for its own troops when they are dealing with Afghans or training Afghans, which will put even more distance between the two sides," Rashid told AFP.

Some of the attacks are claimed by the Taliban, who say they have infiltrated the ranks of Afghan security forces, but many are attributed to cultural differences and antagonism between local and US-led allied forces.

"What we identified was that most of them were caused by personal grievances and stress situations," the chief spokesman for NATO's International Security Assistance Force, Brig-Gen Gunter Katz, told AFP.

"Those isolated incidents don't reflect the overall security situation in Afghanistan. As we speak 500,000 soldiers and policemen are working together to contribute to a more secure and stable Afghanistan," he said.

"We are confident that the morale (among international troops) is still good and those incidents will not affect our transition process."

Katz agreed, however, that there had been an increase in green-on-blue attacks this year, up from a total of 21 incidents and 35 deaths in all of 2011.

"Insurgents understand that this type of action is the most effective one, so they tend to use it as much as they can," a Western security source said.

"But apart from that, there is a general feeling of Afghans being fed up with the foreign troops, cultural issues."

On Tuesday, an American soldier died in the east when two men in Afghan army uniform opened fire, and on Thursday an Afghan soldier was killed after turning his weapon on NATO troops, also in the east.

Then on Friday an Afghan police officer opened fire on four American soldiers he had invited for a meal, killing three of them, Afghan officials said.

The Taliban claimed that attack and said the shooter had fled and joined their ranks. Katz said that the shooter had been detained, without giving further details.

Also on Friday, an Afghan civilian employed on a NATO base shot dead three international coalition soldiers, ISAF said.

"The shooter was not in uniform and our current reporting indicates he was a civilian employee authorised to be on the base, but there is no indication he was an Afghan service member," an ISAF spokesman said.

The spokesman said he had no information on how the civilian got hold of a weapon on the shared Afghan-NATO base in southern Afghanistan, and provided no further details.

President Hamid Karzai "strongly condemned the killing of six NATO soldiers in two separate incidents," attributing them to "terrorists wearing military uniforms", his office said in a statement.

There have also been instances when members of the Afghan security forces have targeted local colleagues.

On Saturday 11 Afghan policemen were killed when one of their colleagues, believed to be a Taliban infiltrator, opened fire on them, officials said.

The incident happened in Delaram district of western Nimroz province.

"Initial investigation shows the shooter was a Taliban infiltrator. He was also killed when police returned fire," Nimroz governor Abdul Karim Brahawi told AFP.