ANKARA, Turkey (AP) — U.S. Secretary of State John Kerry has told his Turkish counterpart that Syrian Kurdish forces have started withdrawing east of the Euphrates River, Turkish officials said Thursday.

The pullback was a major demand by Ankara after Turkey sent in tanks and special forces backed by U.S. airstrikes across the border to help Syrian rebels take a key Islamic State stronghold the previous day.

Turkey's incursion into northern Syria was also meant to contain an expansion by Syria's Kurds amid the neighboring country's civil war, now in its sixth year.

According to Turkish ministry officials, Kerry and Mevlut Cavusoglu spoke by telephone on Thursday to discuss the Turkish military operation to retake the IS-held border town of Jarablus in northern Syria.

Kerry stressed that the Syrian Kurdish forces "were in the process of retreating east of the Euphrates," the officials said in reference to the Syrian Kurdish forces. They spoke on condition of anonymity in line with government regulations

Battling IS militants in Syria, the U.S.-backed Syrian Kurds have been able to seize nearly the entire stretch of the border with Turkey in northern Syria. Turkey has been deeply concerned by the advances of the Kurdish forces, fearing they aim to set up a Kurdish entity there. Ankara maintains that the Syrian Kurdish militia is linked to Kurdish rebels waging an insurgency in southeastern Turkey.

Vice President Joe Biden, who flew into Ankara hours after the offensive on Wednesday, warned that the Syrian Kurdish forces will lose U.S. support unless they retreat east of the Euphrates.

On Thursday, at least 10- more Turkish tanks were seen crossing into Syria at the Turkish border town of Karkamis, the private Dogan news agency reported.

The Turkish assault, launched in retaliation after a string of militant bombings in Turkey, adds yet another powerhouse force on the ground in an already complicated war.

A senior Turkish official told reporters that operations would continue until "we are convinced" that imminent threats to Turkey are neutralized. He said the aim is to create a "terror-free zone" in northern Syria to prevent militants from entering Turkey. The official also spoke on condition of anonymity in line with government regulations.