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Threatening that U.S. Vice President Joe Biden "will be history for me if he has indeed used such expressions," Turkish President Recep Tayyip Erdogan demanded an apology for comments Biden made during a Thursday speech at Harvard's Kennedy School.

"(Erdogan) said: 'You were right. We let too many people through,'" Biden told the audience on Thursday, referencing ISIS extremists who may have traveled through Turkey to Iraq and Syria. "Now they're trying to seal their border."

Biden went on to say that in the fight against ISIS, "our biggest problem is our allies," reports The Associated Press.

The vice president said that other countries in the Middle East were so focused on ousting Syrian President Bashar Assad that they inadvertently helped arm extremist elements as well. Erdogan angrily countered those claims, saying Turkey had stopped 6,000 jihadists from crossing its borders and that it had deported another 1,000 extremists. Turkey has yet to state what its involvement will be in the U.S.-led coalition of countries attempting to stop the spread of ISIS militants through Iraq and Syria.