Sen. John McCainJohn Sidney McCainCindy McCain says no one in Republican Party carries 'voice of reason' after husband's death Anti-gun violence organization endorses Kelly's Senate bid McCain's family, McCain Institute to promote #ActsOfCivility in marking first anniversary of senator's death MORE (R-Ariz.) on Monday hit back against President Obama's claim that the U.S. intelligence community underestimated the threat from ISIS

"This idea that somehow we didn’t know this was happening, of course we knew it, I was there," McCain said on CNN, referring to his trips to the region.

Obama, in an interview on CBS's "60 Minutes" that aired Sunday, said, “Our head of the intelligence community, Jim Clapper, has acknowledged that, I think, they underestimated what had been taking place in Syria.”

McCain, a frequent critic of Obama, again emphasized that the U.S. should have left some troops in Iraq instead of withdrawing in 2011. "It is a direct result of our failure to leave a residual force behind," he said.

While U.S. strikes have hit ISIS in Syria, as well as the al Qaeda branch called the Khorasan Group, the U.S. has not struck President Bashar Assad's forces.

McCain argued that the U.S. should set up a no-fly zone to limit Assad's air force. "We need a no-fly zone," The Arizona Republican said. "If he breaches it, we take out his air force," he added. 

The Pentagon said Friday that it has not ruled out a no-fly zone.

"Assad is responsible for the deaths of nearly 200,000 people," McCain said. "He’s one of the great war criminals there is."