KABUL, Afghanistan -- An Afghan believed to be a police officer shot and killed three coalition soldiers late Friday at a military base in the Garmsir district of restive Helmand province, coalition officials confirmed Saturday.

It was the second fatal attack by suspected members of Afghanistan’s security forces on coalition troops in less than 24 hours and the fourth attack in less than a week.

The attack took place around 9 p.m. at a joint Afghan-ISAF facility. The attack earlier Friday, which killed three U.S. service members, also took place in Helmand, in the Sangin district. The assailant in that case also was wearing an Afghan uniform.

While the identities of the assailants in both cases have yet to be confirmed, the incidents appear to be the latest in an accelerating series of so-called green on blue attacks by local forces on their coalition counterparts that has deepened concerns about the reliability and integrity of Afghanistan’s security forces, who will assume responsibility for security of their country when most coalition combat troops withdraw by the end of 2014.

U.S. Air Force Major Lori Hodge, a spokeswoman for the International Security Assistance Force, as the coalition is formally known, told McClatchy the attackers responsible for both killings had been detained. Hodge said the Sangin shooting had occurred around 1a.m, but declined to comment on media reports that the shooter was an Afghan police commander who killed three U.S. marines after inviting them for a meal.

“All I can tell you is that he was in an Afghan uniform,” she said.

Hodge said she understood that the person who shot the three ISAF soldiers at Garmsir was “an Afghan working for the Afghan Uniformed Police.”

ISAF refused to comment on other details of the Garmsir attack, including the nationality of the dead soldiers or the motive behind the attack. Some news reports said that the gunman opened fire at the coalition soldiers after an argument at the base about access to a gym.

“Afghan and ISAF officials are investigating (the Garmsir shooting) right now to determine the circumstances,” Hodge said.

The U.S. military and ISAF have consistently sought to downplay the significance of green on blue attacks, claiming most are carried out not by Taliban infiltrators but by Afghans who are frustrated with the behaviour of their Western counterparts. One coalition official, who spoke only on the condition of anonymity because of the sensitivity of the issue, said the reasons behind such incidents were complex, but called the recent uptick in attacks “clearly disturbing.”

ISAF has recorded about 69 green on blue attacks since January 2007. Forty-seven such incidents, more than two-thirds, have occurred since the beginning of last year.

On Tuesday, two men wearing Afghan National Army uniforms shot a U.S. soldier to death in eastern Paktia province. On Thursday, an Afghan soldier opened fire on a group of coalition troops outside a military base in eastern Afghanistan.

No international forces were killed in that incident, but the coalition reported that the attacker was killed when its forces returned fire.

Thirty-four coalition soldiers have been killed in 26 green-on-blue attacks this year. Last year, 35 were killed in 21 incidents, according to coalition figures.