(CBS News) - A gunman wearing an Afghan uniform has shot to death three American troops in the southern Helmand province, according to the U.S.-led international military coalition in the country, known as ISAF.
Afghan sources in Helmand tell CBS News Kabul bureau chief Fazul Rahim that the three slain troops were members of a U.S. Special Forces team overseeing the training and recruitment of Afghan Local Police. The sources say it was one of the police officers who opened fire on his mentors during dinner on Thursday evening. ISAF has not yet confirmed these details. It was not immediately clear whether the gunman had fled, or been captured or killed.
The Thursday evening shooting could bring the number of fatal "green-on-blue" incidents - in which Afghan security forces turn on their Western allies - to 24 over the last 12 months, according to ISAF.
The military alliance has yet to confirm that the gunman behind Thursday's attack was an actual member of the Afghan forces, and not just a militant who had acquired a uniform to get nearer his targets. ISAF only counts attacks by serving members of the Afghan security forces as green-on-blue incidents.
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If confirmed, Thursday's will have been the second such attack this week. On Tuesday, two gunmen confirmed to have been Afghan National Army soldiers turned their weapons on NATO troops, killing a service member and wounding two others, U.S. and coalition officials said. The nationality of the slain troop was not immediately confirmed, but was not believed to have been American.
The Taliban claimed responsibility for that shooting, which took place in eastern Afghanistan. The suspects escaped after the attack and a search was underway to track them down and arrest them.
In a separate attack Wednesday morning, at least one suicide bomber walked up to a group of U.S. troops on foot patrol near the provincial council office in Kunar province, also in the east, and blew himself up, killing three American troops. There has been no indication that the bomber or bombers in this attack were wearing Afghan uniforms.
The Taliban traditionally steps up attacks in August, but the militant group's claimed success in recruiting rogue forces from the ranks of the Afghan army and police to turn on their Western allies, with whom they work and live very closely as ISAF ramps up its planned transition of security to Afghan forces, has fostered a lack of trust.
While the Taliban often claim they have infiltrated the Afghan security forces and are carrying out these attacks, CBS News reporter John Bentley reported in July that a U.S. Defense Department report maintains the attacks are not carried out by insurgents.
"Investigations have determined that a large majority of green-on-blue attacks are not attributable to insurgent infiltration of the ANSF (Afghan National Security Forces), but are due to isolated personal grievances against coalition personnel," the report said. "There is no indication that these recent attacks are part of a deliberate effort by insurgents, nor were they coordinated with each other."
The report went on to say that recruiting polices for Afghan soldiers and police had been tightened in an effort to keep disgruntled Afghans from joining.
Early in 2012, top U.S. commander Gen. John Allen ordered American units to select a "guardian angel" to watch over fellow troops, even as they sleep, at joint U.S.-Afghan bases and on joint operations involving live fire - a direct response to the mounting green-on-blue attacks.