In what Sen. John McCain termed "not a very pleasant exchange," the Arizona Republican pressed Defense Secretary Ash Carter over the mission of Syrian rebels being trained by U.S. forces.

"Is it true that with these people that you are training and equipping to fight in Syria, is it true that you are telling them to fight ISIS and not Bashar Assad? Is that true?" McCain asked Tuesday during a Senate Armed Services Committee.

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Carter responded that the U.S. is arming and training them to go after the Islamic State of Iraq and Syria "and not the Assad regime."

"That's our priority and these are people who are inclined in that direction and come from areas overrun by ISIL," Carter said.

He added that the decision on whether they fight the Assad regime as well will be made when fighters enter the field.

"That's of small comfort to those people you're recruiting right now that that decision will be made later on," McCain said. "Is that fair to these young men to say we are sending you in to fight ISIS only and by the way we will decide on the policy whether to defend you if you are barrel-bombed?"