An Afghan working on an installation shared by Afghan and foreign forces shot and killed three NATO soldiers on Friday - raising to six the number of international troops killed by their Afghan partners in 24 hours, officials said.

The attack announced Saturday comes as the number of these so-called "green-on-blue" incidents - carried out by Afghan police or soldiers or those disguised in their uniforms - is on the rise. But the U.S.-led coalition has said that they are not impeding ongoing work to hand over security to Afghan forces by the end of 2014.

The Taliban claimed responsibility for both attacks Friday in Helmand province of southern Afghanistan - an area where Taliban insurgents have wielded their greatest influence.

The assaults have heightened mistrust between international troops and the Afghan soldiers and police they are training and mentoring. The attacks also raise doubts about the quality of the Afghan forces who have already started taking over for foreign forces in areas of the nation that are home to 75 percent of the population.

In the first attack, an Afghan police officer shot and killed the three Marines after sharing a meal with them before dawn in the volatile Sangin district. A U.S. Defense Department official confirmed that the dead Americans were Marine Special Operations Forces.

Later in the day, the "latest information available" indicated that an Afghan working on a joint installation farther south in the province shot and killed three other international troops, said Maj. Lori Hodge, a spokesman for the coalition in Kabul. NATO usually leaves it to member nations to announce the nationalities of their own casualties.

It was not clear whether the shooter was wearing a uniform.

There have been 26 "green-on-blue" such attacks so far this year, resulting in 34 deaths, Hodge said. Four of the attacks have occurred in the past week.

She said the shooters in both attacks have been detained.

Daud Ahmadi, a spokesman in Helmand province, said the second incident occurred at 8 p.m. Friday in Garmser district. He said the three NATO service members were killed and a fourth was injured in the attack. A delegation from the Helmand governor's office was sent to the district to investigate, he said.

Taliban spokesman Qari Yousef Ahmadi told The Associated Press that a member of the Afghan security forces in Garmser was responsible for killing seven international troops, but the insurgency often exaggerates the death toll of its attacks.