The Pentagon is only training 60 Syrian opposition rebels in its plan to fight against the Islamic State in Iraq and Syria, Defense Secretary Ash Carter said Tuesday.

Carter revealed the number during a Senate Armed Services Committee hearing and acknowledged it was a disappointing figure.

"I expect those numbers to increase. But I wanted to tell the truth," he told senators.

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The low numbers drew criticism from members of the Senate panel and could add to the growing skepticism of the program, which is a central part of the administration's strategy to defeat the terrorist group in Syria without having to send in U.S. ground forces.

The plan, which envisions fielding 15,000 rebels over three years, was approved by Congress in September and funded in December, but it took five months to set up the training sites in four countries, develop the training plan and get the first rebels into training.

Defense officials say they hope to train 3,000 by the end of the year and 5,400 by next May, but lawmakers noted that that that timeline is looking increasingly unlikely.

"The math doesn't work," said Sen. Lindsey Graham (R-S.C.), a senior member of the committee and 2016 presidential candidate.

Carter said that there were 7,000 volunteers waiting to be vetted and that he hoped that the numbers would increase.

The main factor keeping the number of trainees down is the "rigorous" vetting of the volunteers, he said.

Carter said volunteers must not be affiliated with ISIS or any other extremist group and must not have committed atrocities. The U.S. also must be certain that the recruits will not attack their U.S. trainers, and that they will be willing to go after ISIS instead of Assad regime forces, who they are also fighting.

"That's why 60 of the them got out the other end of the process," he said.

Sen. John McCain (R-Ariz.), who has championed supporting the Syrian rebels in their four-year insurgency against Syrian President Bashar Assad, blasted the low numbers.

"I've got to tell you after four years, Mr. Secretary, that's not a very impressive number," he said.

McCain also criticized the Pentagon for making rebels promise not to target Assad's forces and not yet deciding on how to support Syrian rebels when they head into battle against ISIS.

"That's of small comfort to those people you're recruiting right now, that that decision will be made later on," he said.