The killings are the latest in a series of "green-on-blue" attacks carried out by Afghans clad in security uniforms, and they follow a claim purportedly from Taliban leader Mullah Omar that fighters are infiltrating Afghan security forces to attack NATO-led forces on their bases.
"As a result, the foreign invaders and their allies at their military centers and bases are suffering crushing blows by these heroic soldiers."
CNN can't independently verify the authenticity of the statement, which was released in advance of Eid al Fitr celebrations this weekend that mark the end of the Muslim holy month of Ramadan.
The Taliban is known to routinely claim responsibility for attacks and inflate casualty numbers.
Omar also urged employees of the Afghan government to seek out the group's newly established "Call and Guidance, Luring and Integration" departments that have been established throughout the country "so that they may leave the enemy ranks and join the fighters," according to SITE.
Omar's statement drew a sharp response from U.S. Army Gen. John Allen, commander of NATO troops in Afghanistan, who accused the Taliban leader of issuing "an unmistakable message of death, hate and hopelessness for the Afghan people" on the eve of the Eid celebrations.
"The pride of the Afghan people has been smeared by killers who pose as soldiers and police, yet they represent the worst of humanity," Allen said Friday.
The commander accused of Omar of sending "young brainwashed men to carry out attacks in a fruitless cause," while he "rests comfortably from afar."
"He professes love for his fighters, yet he sends them to their deaths by the hundreds. Where is the vision that Omar speaks of? Where is the love he professes for the Afghan people?" Allen said.
"Are these not the acts of a deranged man who puts his own goals of personal domination ahead of the future of the Afghan people?"
Afghan security personnel have killed an estimated 93 NATO soldiers since May 2007, and they have killed at least 31 this year alone across the country, said Bill Roggio, managing editor of the Long War Journal blog, which reports and analyzes terror issues. That figure does not include the two deaths in Farah province Friday.
The NATO-led International Assistance Security Force did not provide details about the attack, but a provincial police chief told CNN that an Afghan policeman shot the American service members early Friday during a training session in the Bala Boluk district.
Police Chief Aqa Noor Kintos said two U.S. Special Forces members were killed and another was injured. The gunman was killed when troops returned fire, he said.
During the gunfight, an Afghan National Police member was also killed and another was injured, he said.
Lt. Col. Hagen Messer, an ISAF spokesman, confirmed that two U.S. service members were killed in the attack.
The Taliban claimed responsibility in a text message to journalists, saying a 70-year-old policeman killed the three troops.
It's the second time a man in an Afghan security uniform opened fire on ISAF troops in the Bala Boluk district. In December 2011, a gunman posing as an Afghan soldier wounded coalition troops.
The frequency of the green-on-blue attacks has prompted Afghan intelligence agents to begin going undercover during recruit training to spot possible extremists, military officials say.
U.S. Army Gen. John Allen, commander of NATO troops in Afghanistan, said coalition officials were working on a new procedure to check the backgrounds of Afghans who sign up for the army or police force.
The U.S. government has offered a reward of up to $10 million for information leading to the arrest of Omar, who took over Afghanistan in the early 1990s and established a hard-line Islamic fundamentalist regime that gave shelter to Osama bin Laden and his al Qaeda terrorist network.
Omar vanished from public view after a U.S.-led coalition ousted the Taliban and its leaders from power in Afghanistan in December 2001 for refusing to hand over bin Laden after the 9/11 terror attacks.
Over the years, he has refused to be photographed or filmed, and has rarely traveled. As a result, Omar's appearance has remained a mystery to many. Those who have met Omar say he has one eye stitched shut, the result of a wound suffered during a battle with Soviet troops during their occupation of Afghanistan.
There have been conflicting reports about the fate of Omar. As late as last year, the Taliban was denying reports that its leader was dead, though he has not been seen in public since the Taliban was ousted from power.