The Pentagon says dozens of Afghan troops who had been visiting the United States for military training have gone missing since January 2015, presumably to live and work illegally in the United States.

Defense Department spokesman Adam Stump told Reuters that the whereabouts of 44 Afghan troops are unknown.

Since September 1, eight Afghan troops have left military bases without authorization.

An unidentified U.S. defense official said the frequency of Afghan troops going missing was concerning and "out of the ordinary."

The Pentagon is “assessing ways to strengthen” screening procedures for the training program, Stump said.

Some 2,200 Afghan troops have received military training in the United States since 2007.

Since 2002, the United States has allocated more than $60 billion to train and equip Afghan security forces.

Fifteen years after the start of the U.S.-led invasion that toppled the Taliban regime, Afghanistan remains one of the world's poorest and most politically unstable countries.

Based on reporting by Reuters