The NATO-led command did not provide details about the deadly attack in Farah, 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, a NATO-led command 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 has opened fire on coalition troops in the Bala Boluk district. In December 2011, a gunman posing as an Afghan soldier wounded coalition troops.
In the Kandahar incident, the member of the Afghan National Army who turned his weapon against coalition forces was shot and later died of his wounds after he was taken to an ISAF medical facility. The incident occurred in the province's Zharay district, ISAF said.
The frequency of the insider attacks has prompted Afghan intelligence agents to go undercover during recruit training to spot possible extremists, military officials said.
Allen 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 al Qaeda.
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 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 Omar's fate. As late as last year, the Taliban denied reports the leader was dead.
The attacks come a day after a helicopter crash in southern Afghanistan killed seven U.S. service members, three Afghan soldiers and an Afghan interpreter. Authorities are investigating whether the crash was caused by technical problems or a shoot-down.