The Kurdish-backed Syrian Democratic Forces allegedly began moving east across the Euphrates river as Turkey said it had “every right” to intervene if Syrian Kurdish militia do not take that move, as promised by the United States.

The Syrian Kurdish forces “must move to the east of the Euphrates and Turkey is following this very closely,” Defense Minister Fikri Isik told NTV television.

He was speaking a day Ankara launched an operation in Syria on Wednesday aimed both at ISIS and Syrian Kurdish forces.

“If this withdrawal doesn’t happen, Turkey has every right to intervene,” Isik added.

Turkey has said the operation in Syria is aimed not just at ISIS jihadists but also the Syrian Kurdish Democratic Union Party (PYD) and its People’s Protection Units (YPG) militia.

Turkey sees the YPG as a terror group bent on carving out an autonomous region in Syria.

Ankara’s hostility to the YPG puts it at loggerheads with its NATO ally, the United States, which works with the group on the ground in the fight against ISIS.

U.S. Vice President Joe Biden, visiting Turkey on Wednesday, made clear that Washington has strictly told the YPG not to move west of the Euphrates and would no longer receive American support if they did.

“They have not yet withdrawn but we are watching and monitoring whether they will withdraw. Turkey will be following, moment by moment,” Isik said, adding the withdrawal was promised within a week.

But a spokesman for the U.S.-led coalition against ISIS had tweeted that the Syrian Kurdish forces “have moved east across the Euphrates to prepare for the eventual liberation” of the ISIS stronghold of Raqqa.

According to Turkish ministry officials, U.S. Secretary of State John Kerry and his Turkish counterpart Mevlut Cavusoglu spoke by telephone on Thursday to discuss the Turkish military operation to retake the ISIS-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.

The Britain-based Syrian Observatory for Human Rights said only a small number of the Kurdish forces had moved east back across the river, and most were still on the western side.

The Syrian Democratic Forces had moved west of the Euphrates as part of the U.S.-backed operation to capture the city of Manbij from ISIS. The Kurdish forces were still present around Manbij, which lies well west of the Euphrates, the Observatory said.

But the YPG told Agence France Presse in Beirut it had no interest in listening to ultimatums laid down by Turkey.

“The YPG are Syrians and they are present on Syrian land — Turkey cannot impose restrictions on the movements of Syrians on their land,” said YPG spokesman Redur Xelil.