After many of the first 54 graduates of a special training program were attacked in Syria by an al-Qaeda affiliate in July, only a handful of rebels remain in the fight, General Lloyd Austin told the Senate Armed Services Committee.
"It's a small number," Austin said. "The ones that are in the fight is... We're talking four or five."
President Barack Obama's administration has set up a half-billion-dollar mission to train Syrian opposition fighters and the original aim was to recruit around 5,400 vetted Syrians each year for three years to fight the Islamic State group.
Austin's comments came amid a withering Senate hearing, in which senior lawmakers blasted as a failure the current US efforts to fight the IS group.
Republican Senator Kelly Ayotte said the low number of US-trained rebel fighters was a "joke.