The highly-touted U.S. effort to train and equip a moderate force of rebels to fight the Islamic State in Syria has so far produced only 60 fighters — much less than expected, Defense Secretary Ash Carter said Tuesday.

"This number is much smaller than we hoped for at this point," Carter told the Senate Armed Services Committee, saying the rigorous vetting process had weeded out more potential recruits than expected. The program had been expected to produce some 5,000 trained fighters in a year.

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"It's going to take some time, obviously, to get the numbers up to the point where they can have an effect."

Carter's admission was the latest sign of concern that the Obama administration's strategy against the extremist group in Syria has not lived up to expectations. Syria always has been an afterthought for the administration's approach, behind Iraq, and the extremists continue to hold on there in spite of recent gains by Kurdish forces in northern Syria.

The administration has refused any cooperation with the regime of Syrian President Bashar Assad, but also has not taken any action against him, insisting that there is no military solution to his rule. That has irked Syrian opposition leaders whose forces will be needed to defeat the Islamic State, as well as their supporters in Congress.

In a tense exchange, Committee Chairman John McCain challenged Carter and Joint Chiefs Chairman Gen. Martin Dempsey on that point, saying the United States needs a policy that protects the rebels from the Assad regime as well as the Islamic State.

"It's shameful to send people in and not assure them we will protect them from attacks by barrel bombs," the Arizona Republican said.