A day after Turkish tanks crossed the border into northern Syria, most of a U.S.-backed largely Kurdish army has moved east of the Euphrates River on Thursday, according to a U.S. military spokesman.

Turkey and Vice President Joe Biden had called on the Kurds to leave Arab lands in Syria and return east of the Euphrates to traditional Kurdish territory. Biden threatened to cut off supplies to the Kurds if they didn’t fall back.

Some of the Syrian Democratic Forces remained in the region to remove roadside bombs, U.S. officials added.

The United States is supporting the Turkish military operation with airstrikes and conducted at least eight against ISIS targets on Wednesday, according to a senior defense official.

But the U.S. is also supporting Kurdish fighters in Syria, including a militia known as the YPG, seen as the best ground force against ISIS in Syria.  Some US special operations forces have embedded with the YPG, part of the 300 US troops sent to Syria earlier this year.

Turkey considers the YPG a terrorist organization, the Syrian affiliate of a Kurdish separatist group in Turkey, the PKK. The State Department lists the PKK as a foreign terrorist organization.  The PKK has fought a decades long insurgency against Turkey killing tens of thousands of civilians.  Attacks by the PKK have increased in recent months. Turkish jets have bombed PKK positions in response, including some at training camps in northern Iraq.

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