U.S. President Barack Obama welcomed Turkey’s effort with Russia to revive a Syrian cease-fire and peace talks in a conversation with Turkish President Recep Tayyip Erdogan.

The White House said on January 3 that Obama expressed condolences to Erdogan over a “horrific” terrorist attack on December 31 in an Istanbul nightclub claimed by the Islamic State (IS) extremist group as well as a December 17 attack on Turkish police in Kayseri that was blamed on an outlawed Kurdish group.

The two leaders noted progress in their campaign against Islamic State in Iraq and Syria and agreed that “Turkey and the United States must continue to stand united in order to defeat terrorism,” the White House said.

The conversation came after recent accusations from Erdogan and his deputies that the United States has been arming “terrorists” in Syria in an apparent reference to U.S. aid given to Syrian Kurdish militias who have been battling IS.

Turkish Prime Minister Binali Yildirim on January 3 urged U.S. President-elect Donald Trump to put an end to “this shame” and questioned whether the U.S.-backed Kurdish forces were doing anything to defeat IS.

“They are pretending to fight IS. Turkey is the only country that is leading a fight. The United States isn’t doing anything,” Yildirim said.