Story highlights President Barack Obama hit Donald Trump over his comments about NATO

Obama also said Trump is 'ultimately helping do ISIL's work for us'

Philadelphia (CNN) President Barack Obama had strong words for Donald Trump's rhetoric over Muslims in the wake of terrorist attacks, saying -- using another acronym for ISIS -- that Trump "is ultimately helping do ISIL's work for us."

Obama also said Trump's suggestion he might not back NATO allies show he's unprepared to handle foreign policy, in an interview aired Sunday.

Obama's comments came in an interview with CBS' John Dickerson on "Face the Nation." The day before the Democratic National Convention's kickoff, the President made clear he's eager to attack Trump over foreign policy.

He lambasted Trump for suggesting the United States should refuse to assist NATO allies who haven't contributed enough financially to the trans-Atlantic alliance.

"I think that anybody who's been paying attention knows there is a big difference between challenging our European allies to keep up their defense spending, particularly at a time when Russia's been more aggressive, and saying to them, 'You know what? We might not abide by the central tenant of the most important alliance in the history of the world,'" Obama said.

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