Ssix American soldiers have been killed in rogue attacks.
The shooting took place on Friday night in the Garmsir district of Helmand province, where three US special forces soldiers were killed by an Afghan policeman and comrades earlier in the day.
"Let me clearly say that those two incidents clearly do not reflect the overall situation here in Afghanistan," the chief Nato force spokesperson, Brigadier-General Gunter Katz, told reporters on Saturday.
The three Marines were shot by a base employee who turned a gun on them, in the third rogue attack in four days. Foreign military sources said the man had not been wearing a uniform and it was unclear how he got hold of the weapon.
The gunman had been detained and a joint Afghan-Nato investigation team was reviewing security and looking into the reason for the attack.
In the earlier attack, an Afghan police commander and several of his men killed three US Marines in darkness early on Friday after inviting them to a Ramadan breakfast to discuss security.
The three men were all Marine Corps special operations forces and appeared to have been killed in a planned attack by rogue Afghan forces. Nato calls such incidents green on blue attacks.
The Nato force says there have been 26 such attacks on foreign troops since January in which 34 people have been killed. Last year, there were 21 attacks in which 35 people were killed.
But a coalition spokesperson said the killings by the Afghan worker would not be included in that tally as it did not involve a member of the Afghan security forces.
Green on blue shootings, in which Afghan police or soldiers turn their guns on their Western colleagues, have seriously eroded trust between the allies as Nato combat soldiers prepare to hand over to Afghan forces by 2014, after which most foreign forces will leave the country.
Summing up mood
But Katz said the incidents were relatively isolated and were not hurting cooperation between foreign forces and the 350 000-strong Afghan Security Forces.
"We have almost 500 000 police and soldiers working together, side by side, enhancing their trust and enhancing their cooperation in order together to fight for a better future for this country," he said.
Nato has directed its forces to increase measures against rogue attacks, including placing armed "guardian angel" soldiers on duty in areas where troops gather, such as gyms and meal halls. Soldiers are also required to travel in pairs in Afghan base areas and carry weapons at all times.
The majority of rogue shootings, Katz said, were due to personal disagreements between Afghan forces and their Western mentors, or were due to combat stress, rather than successful infiltration of the security forces by insurgents.
The Afghan military has also placed intelligence agents within Afghan units to watch for signs of rogue attacks, with dozens of Afghan police or soldiers moved to other bases following disagreements with foreign soldiers.
Mood of Afghan
"Together with our Afghan partners we look into procedures, how to mitigate these incidents," Katz said.
Last month, an Afghan policeman opened fire on British soldiers in Helmand province, killing three in an attack claimed by the Taliban, and a gunman in uniform killed foreign trainers working for Nato in western Herat province, killing three.
A Taliban statement posted on Twitter said the attacks were "clearly summing up mood of Afghan nation towards foreign occupation".
Violence in Afghanistan is at its fiercest since US-led Afghan troops overthrew the Taliban government in 2001. Insurgents have extended their reach from traditional strongholds in southern and eastern areas to parts of the country once considered safe.
At least six civilians were killed on Friday in Helmand when a roadside bomb blew up their vehicle. – Reuters