Vice President Mike Pence is being sent to the Middle East by President Donald Trump as U.S. troops pull out of northeast Syria and Turkish forces invade.
Pence says Trump spoke with Turkish leader Recep Tayyip Erdogan earlier Monday and called for an immediate end to Turkey's moves against the Kurds in Syria.
Pence will reportedly meet with Erdogan on Thursday in Ankara.
Pence says the president is "very concerned about instability in the region" and is denying that Trump gave Turkey the green light to launch the invasion when he announced a troop pullout.
Pence says the U.S. is "simply not going to tolerate Turkey's invasion of Syria any longer."
Trump has slapped new sanctions on Turkey to pressure the country to halt an invasion that has opened the door to a resurgence of the Islamic State group.
Treasury Secretary Steven Mnuchin said the sanctions will hurt an already weak Turkish economy. Pence said the U.S. will continue to ramp up the sanctions "unless Turkey is willing to embrace a cease-fire, come to the negotiating table and end the violence."
The Associated Press contributed to this article.