AP
Joe Biden is coming under criticism from Turkish President Erdogan.
Speaking at Harvard University on Thursday, Biden told students Erdogan - who he referred to as an "old friend" - admitted to him that his country had erred in allowing foreign terrorist fighters an easy route to pass in and out of Syria.
"President Erdogan told me - he's an old friend - he said, 'You're right. We let too many people through.' Now, they're trying to seal their border," Biden said.
Erdogan said he never made such an admission to Biden, and he said Biden would be "history for me" if he does not apologize.
"I have never said to him that we had made a mistake, never. If he did say this at Harvard then he has to apologize to us," Erdogan said, according to The Associated Press.
"Foreign fighters have never entered Syria from our country. They may come to our country as tourists and cross into Syria, but no one can say that they cross in with their arms."
But closing Turkey's porous border has indeed been a major point of emphasis in the fight of the US and partners against the extremist group calling itself the Islamic State, also known as ISIS or ISIL. It is routinely brought up as a point of concern throughout the Obama administration. Erdogan has taken steps to enhance security along the border in recent months.
AP Photo Erdogan
The dispute between Biden and Erdogan threatens to upset a key flank in the alliance against ISIS at a delicate moment. ISIS militants and Kurds are battling along the Syria-Turkey border in the key strategic city of Kobane, a situation that has led more than 150,000 refugees to flee from Syria to Turkey in the past two weeks alone.
Turkey's parliament this week expanded the government's power to authorize military operations against ISIS, voting to deploy ground troops into the conflict (though no deployment is imminent) and allow NATO allies and other coalition partners to use Turkish airspace in the fight.
In his question-and-answer session with students at Harvard, Biden blamed the US' allies in the Middle East for the deteriorating situation in Syria's civil war and for the rise of extremist groups like ISIS.
He said countries like Turkey, Saudi Arabia, and the United Arab Emirates were too focused on ousting Syrian President Bashar al-Assad that they did not properly vet the opposition groups to which they sent money and weapons.
"The Turks, who are great friends - I have a great relationship with Erdogan, whom I spend a lot of time with. The Saudis, the Emiratis, etc. What were they doing?" Biden said.
They were so determined to take down Assad and essentially have a proxy Sunni-Shia war, what did they do? They poured hundreds of millions of dollars and thousands of tons of weapons into anyone who would fight against Assad - except that the people who were being supplied were al-Nusra and al-Qaeda and the extremist elements of jihadis coming from other parts of the world."
Biden and Erdogan spoke over the phone Friday afternoon, the White House said. The White House's readout of the call did not include any mention of disagreement, only saying the two "discussed ways the United States and Turkey can work together to degrade and destroy" ISIS.
Biden's office did not immediately respond to a request for comment Saturday morning on Erdogan's request for an apology.