WASHINGTON (AP) — The U.S. hopes to boost a faltering program for training moderate Syrian rebels to fight the Islamic State, but it will not compromise on the standards it has used to weed out problematic volunteers, Pentagon leaders said Wednesday.

The vetting has been so strict that of an estimated 6,000 Syrian volunteers, only 1,500 have been declared qualified so far and, of those, fewer than 100 have been retained in the training taking place at bases in Jordan and Turkey.

The numbers are so low that critics question whether a meaningful moderate rebel ground force can be built in time to make a difference against the Islamic State.

Asked about the slow pace of the program, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told a Pentagon news conference that he is hopeful that it can be accelerated. He gave no indication that major changes are in the offing.

"We need credible, moderate partners on the ground. So we are always looking for the opportunity to develop those partnerships," he said, standing beside Defense Secretary Ash Carter. "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 suggested that the Muslim holy month of Ramadan, began in mid-June, has put a damper on volunteers.

"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, referring to dozens of Syrian trainees recently quitting.

The U.S. goal is to train and equip 5,400 rebels per year for three years.

Carter gave no indication that he is considering changing the U.S. approach to the training program.

"It's difficult to get large numbers trained at the other end, although we do hope and expect that those numbers will increase," Carter said. "We want to accept more and I expect in time will be accepting more."