Fewer than 100 rebel volunteers have so far been trained to take on Islamic State (Isis) on the ground in Syria, the Pentagon said.
Of 6,000 Syrians who volunteered, only 1,500 were declared qualified to undergo training after vetting, and fewer than 100 of these retained in the training taking place in Jordan and Turkey, General Martin Dempsey, chairman of the Joint Chiefs of Staff, told a Pentagon news conference.
Dempsey told reporters he hoped the pace of training could be accelerated, with critics arguing air strikes alone will not be enough to dislodge IS from Syria, where it is believed to control more than 50% of territory.
"We need credible, moderate partners on the ground. So we are always looking for the opportunity to develop those partnerships," he said.
"We certainly won't take any shortcuts on vetting, however, because of the risk that would pose not only to our own forces, but to the ultimate objectives we are trying to achieve."
Dempsey said numerous recruits had walked out of training over the Muslim holy month of Ramadan.
"There's a lot of folks that are interested in being with their families during that period, and so we may see after Ramadan that some of the ones we lost may come back," he said.
He gave no indication that the training programme would change to increase the numbers being successfully trained.
The US began training Syrian moderate rebels in Turkey in June. The Pentagon aims to train 5,400 fighters a year over three years.