(CNN) The United States has only trained approximately 60 Syrian rebel fighters as of July 3, Defense Secretary Ashton Carter told the Senate Armed Services Committee on Tuesday, saying the number is "much smaller" than the administration hoped to train at this point.

"I said the number 60, and I can look out at your faces and you have the same reaction I do, which is that that's an awfully small number," he said.

Carter's admission highlights the increasing concern over the effectiveness of a program to train a local fighting force to combat ISIS in Syria.

The low numbers are blamed on a strict vetting process that includes ensuring the fighters are committed to combat ISIS, as opposed to the Assad regime, and passing a counter-intelligence screening.

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"We make sure that they, for example, aren't going to pose a green-on-blue threat to their trainers; that they don't have any history of atrocities," Carter said.

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