Three coalition troops — two British and one American — were killed Monday by Afghan security forces in two separate incidents.

NATO officials say that in the first incident, two British soldiers were shot dead at a NATO base in Lashkar Gah in southern Helmand province. Coalition troops returned fire, killing the attacker, who was said to be wearing an Afghan army uniform.

Hours later, another coalition service member — an American — died following a shooting incident in eastern Paktika province. NATO says the soldier was shot by a member of the Afghan local police while approaching a police checkpoint.

Afghan and coalition forces are investigating the circumstances of both attacks.

U.S. officials say there have been at least 42 attacks by Afghans on their NATO colleagues since 2007. U.S. Defense Secretary Leon Panetta said Monday that despite a recent series of killings of NATO troops by Afghan soldiers, such violence is sporadic. He said the attacks should not detract from the main strategy, which is to transfer control of security to the Afghans.

Monday's incidents come two weeks after a U.S. soldier, Army Staff Sergeant Robert Bales, allegedly went on a shooting spree in Helmand, killing 17 Afghan civilians.

The top commander in Afghanistan, U.S. Marine Corps General John Allen, said Monday he is looking into possible leadership failures that might have led to the shootings. Speaking to reporters at the Pentagon, General Allen suggested a major probe is under way in the wake of high-profile incidents, including the recent inadvertent burnings of the Quran by U.S. troops.

Bales, an 11-year military veteran, is being held at a U.S. military prison in the midwestern state of Kansas and faces 17 counts of murder and six counts of attempted murder. He is alleged to have walked out of his southern Afghanistan military base under the cover of darkness March 11, gunning down nine children and eight adults in nearby villages.