​U.S. and Syrian rebel allies at odds over who is the enemy

ISTANBUL -- The Pentagon's first attempt to train and equip Syrian rebels to fight ISIS was what one official called "abject failure." The initial 54 fighters are either dead, captured or missing, and never came in contact with ISIS.

Now a Syrian opposition source tells CBS News another 600 fighters -- all of whom passed the U.S. vetting process -- have also dropped out.

America's hopes of defeating ISIS in Syria depend on fighters like Abu Suleyman and Abu Yazen, battle hardened 22-year-olds, both too frightened to show their faces because their parents still live under ISIS rule.

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Syrian rebels Abu Suleyman and Abu Yazen CBS News

In April, along with other fighters from their brigade of so-called "moderate" rebels, they signed up for U.S.-funded training.

But after completing the program -- and receiving their first $500 in pay -- they quit.

"We felt betrayed," said Abu Suleyman. "About halfway through, we realized the trainers only wanted us to fight against ISIS and not the Syrian regime."

Both men told us they've risked their lives battling ISIS, but like many rebels they believe the bigger enemy is the Syrian government, which continues to bomb its own people.

Early U.S. training of Syrian rebels seen as "failure"

The 54 who completed the training and returned to the war zone were quickly attacked by an al Qaeda linked group.

Captain Ammar al Wawi is an officer in the division to which the 54 belonged and told us five were killed, and 13 were captured.

"They had no protection from the air, and they were surrounded by terrorists," he said. "They should have waited until they had at least 200 men, so they could defend themselves."

Captain Al-Wawi told us that the U.S.-funded training program is taking so long, that at this rate it will take more 30 years to build a new Syrian Army.