KABUL: An Afghan civilian employed on a Nato base, shot dead three international coalition soldiers on Friday - the same day three others were killed by an Afghan policeman, Isaf said on Saturday.

“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.

Also 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, in the third so-called green-on-blue assault in just four days.

The US military in Afghanistan confirmed that three US soldiers had been killed by “an individual in an Afghan uniform” in Sangin district of southern Helmand province, but gave no further details.

While the latest shooting does not fall directly into the green-on-blue category, which involves Afghan security forces turning their weapons against their Nato allies, it will further erode trust between foreign troops and the Afghans they work with.

More than 30 Nato soldiers have been killed in about 23 green-on-blue attacks this year, according to an AFP tally.