Afghanistan ? The second shooting of Western troops by one of their Afghan counterparts this month has highlighted the potential hazards of a push to speedily expand Afghanistan’s army and police forces in the next few years.

On Tuesday, an Afghan army sergeant opened fire at an army base in northern Afghanistan, killing two American civilian trainers before being shot dead. That followed an attack in the south on July 13, when a soldier killed three British troopers, including the company commander, with gunfire and a rocket-propelled grenade in the dead of night.

Military commanders have described the two attacks as isolated events, and it is indeed rare for an Afghan soldier to turn on NATO forces. Still, they feed on larger doubts about the ongoing massive recruiting among a largely illiterate population — many of whom are used to holding a gun but not to rigid military discipline.

The concerns include possible infiltration by the Taliban and the professionalism of the forces at a time when NATO hopes to expand the Afghan army from 85,000 troops in 2009 to 134,000 by October 2011. The eventual goal is to turn over the responsibility for nationwide security to Afghan forces by 2014 so that foreign troops can go home.