Three U.S. soldiers have been killed by an Afghan worker in southern Afghanistan, bringing the total death toll of a deadly 24 hours for NATO-led troops in the country to six, the BBC reported.

The U.S. Marines were killed at a base in Helmand province on Friday, where three U.S. special forces soldiers were shot dead by an Afghan police officer earlier in the day.

The Taliban has claimed responsibility for both attacks.

NATO calls such attacks green on blue shootings.

At least 34 international coalition soldiers have been killed in 26 such incidents this year, the BBC quoted U.S. military spokeswoman Maj Lori Hodge as saying.

But the chief spokesman for the Nato-led International Security Assistance Force said the increasing number of green on blue attacks, in which members of the Afghan security forces turn their guns on their Western allies, did "not reflect the overall situation" in Afghanistan.