The United States made it clear to Kurdish forces in Syria that retreating beyond the east of the Euphrates river after seizing control of the Syrian town of Manbij is a condition to retain U.S. support, Vice President Joe Biden said on Wednesday.

Ankara has worded its expectations for Syrian Kurdish fighters to withdraw across the river after the Manbij victory by the U.S.-backed Syria Democratic Forces (SDF). Kurdish YPG in Syria is identified as a hostile force by Turkey.

Speaking during a visit to Turkey, Biden said Turkish officials made clear that the rule of law will prevail during an investigation into the July 15 failed putsch. Tens of thousands of arrests were made after the abortive coup.

Turkish-backed Syrian rebels entered ISIS stronghold in the town of Jarablus in northern Syria on Wednesday in an operation supported by Turkish tanks, special forces units and U.S.-led coalition warplanes, Syrian opposition sources reported.

A Syrian opposition commander with the Failaq al Sham group, who requested anonymity, told Reuters that most ISIS fighters in Jarablus had pulled out, some of them surrendering.

Another commander estimated up to 50 percent of the town was now under the control of the Turkish-backed opposition fighters.

“ISIS fighters have withdrawn from several villages on the outskirts of Jarablus and are heading south towards the city of al Bab,” the Failaq al Sham commander said.