KABUL (Reuters) - Three U.S. Marines have been shot dead by an Afghan worker on a military base in southern Afghanistan, in a deadly 24 hours for NATO-led forces during which six American soldiers were killed in rogue attacks.

The shooting took place on Friday night in the Garmsir district of Helmand province, where three U.S. 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 spokesman, 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 reported here, an Afghan police commander and several of his men killed three U.S. 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 spokesman 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.