Ankara: Turkey on Thursday said it had "every right" to intervene if Syrian Kurdish militia do not withdraw east of the Euphrates River in Syria, 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," Defence Minister Fikri Isik told NTV television.
He was speaking a day Ankara launched an operation in Syria on Wednesday aimed both at the Islamic State group (IS) 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 IS 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 IS.
US 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.
But the minister said there was as yet no sign of the withdrawal.
"They have not yet withdrawn but we are watching and monitoring whether they will withdraw.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.
A spokesman for the US-led coalition against IS had tweeted that the Syrian Kurdish forces "have moved east across the Euphrates to prepare for the eventual liberation" of the IS stronghold of Raqqa.
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 Kurdish forces were still present around the Syrian town of Manbij seized from IS earlier this month, which lies well west of the Euphrates, it added.
But the YPG told AFP 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.