(CBS News) LONDON - Two U.S. troops were killed in the western Afghan province of Farah when an Afghan Local Police officer turned his gun on them, one of two so-called green-on-blue attacks to hit NATO forces Friday amid a dramatic escalation of the incidents during the last year.
The police officer was shot and killed, according to NATO, which said an investigation was underway. Local sources told the Associated Press that the shooter had been recruited to the Afghan Local Police - a network of regional militias backed by the international military alliance and trained largely by NATO forces - just five days ago.
In the second attack on Friday, a member of the Afghan security forces shot and wounded three foreign troops in the southern Kandahar province, coalition spokesman Brig. Gen. Gunter Katz tells CBS News reporter Kitty Logan. Katz said the shooter was in custody, but could provide no further details on the incident.
With Friday's attacks, Afghan security forces working in partnership with Western troops have now turned weapons on their allies 31 times in 2012. At least 39 coalition forces have been killed in green-on-blue attacks this year, according to NATO, including 23 Americans.
There were two separate green-on-blue attacks last Friday, also, which left six American troops dead.
The shooting comes one day after a statement released by the head of the Afghan Taliban, Mullah Omar, claiming that militants had successfully infiltrated Afghan security forces to carry out such attacks.
"Thanks to the infiltration of the Mujahideen (holy warriors), they are able to (safely) enter bases, offices and intelligence centers of the enemy," claimed Omar in the statement, released to mark the Islamic holiday of Eid al Fitr, the end of the holy month of Ramadan.
The Taliban often claim responsibility for green-on-blue attacks, but the U.S. Defense Department maintains the attacks are not generally not carried out by insurgents, but rather individual members of the Afghan security forces who may develop a grudge against their Western allies.
Defense Secretary Leon Panetta announced earlier this week that new counter-intelligence measures had been adopted during the past year to try and prevent the green-on-blue attacks. While he acknowledged that some of the incidents have been connected to insurgent groups, he insisted that the vast majority of the attacks appear to be carried out by people with no known links to, or coordination with, the Taliban or other militant organizations.
Even the Taliban leader conceded in his written statement that not all of the attacks were carried out by his militants, but he expressed his appreciation to the "conscious Afghans in the rank and files of the enemy," some of whom, he claimed, had gone on to "join the ranks of Mujahideen, carrying their heavy and light weapons and ammunition."
U.S. military officials note that the majority of the attackers in green-on-blue incidents are actually killed or captured soon after they turn their weapons on their colleagues.
In conjunction with Afghan commanders, the U.S. has used an eight-point vetting process to try and identify and exclude Afghan recruits who pose a threat, but neither Panetta nor other U.S. commanders have elaborated on what news steps are being taken to confront the problem.
The number of international forces killed in green-on-blue attacks in 2012 has already surpassed the toll from 2011, when 35 Western forces were killed.
CBS News correspondent David Martin reported earlier in August that about 84,000 U.S. troops remain in Afghanistan, along with 40,000 from other NATO countries - all working with 332,000 Afghan army and police personnel to try and defeat an enemy which numbers at just 20,000, according to estimates.
Those numbers will change as the U.S. draws down its forces to 68,000 by October, in line with the Pentagon's plans, but Martin notes that the Americans who stay in the country will remain vulnerable to the insider attacks.
Operating side-by-side with Afghan forces is the linchpin of the American strategy in Afghanistan, says Martin, giving them the support they need to take over the fighting so U.S. troops can come home.
The trust between American forces and the Afghan police and soldiers they are training and supporting has been dealt such a blow by the green-on-blue attacks that the Pentagon instituted a "guardian angel" program early this year, whereby members of U.S. military units are picked to watch their fellow troops' backs as they eat, sleep or patrol with Afghan counterparts.