GOP presidential candidate Donald TrumpDonald TrumpTrump wants B for 62 miles of border wall: report Trump can boost national security, own legacy with Russian arms deal Press: Did Trump learn? MORE late Monday offered his approach for fighting with the Islamic State in Iraq and Syria (ISIS).

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"Why are we knocking ISIS, and yet at the same time we're against [Syrian President Bashar] Assad? Let them fight, take over the remnants," Trump said on CNN’s “Out Front,” calling Syria a “total catastrophe.”

"But more importantly, let Russia fight ISIS, if they want to fight them, in Syria," Trump said. "We can fight them in Iraq," Trump added.

"Let ISIS and Syria fight. And let Russia, they're in Syria already, let them fight ISIS," Trump said. "Let Russia take care of ISIS."

President Obama and Russian President Vladimir Putin met privately for 90 minutes Monday to discuss Syria after giving dueling addresses at the United Nations General Assembly in New York.

"Yesterday's meeting was genuinely constructive," Secretary of State John Kerry said Tuesday on MSNBC's "Morning Joe."

The pair discussed issues such as having a unified Syria, that ISIS must be taken on and that there needs to be an organized transition in Syria, Kerry said.

ISIS is considered a threat to Assad, whom the U.S. wants out of power but Putin believes is key to defeating ISIS.