President Recep Tayyip Erdogan said on Friday that Turkey has swept ISIS militants and Kurdish YPG militia from an area of northern Syria, but he rejected claims that the U.S.-backed militia had retreated east of the Euphrates River following Turkish strikes against the group.

Erdogan told a news conference early on Friday that the cross-border offensive into Syria dubbed “Euphrates Shield” had been successful in clearing ISIS and Kurdish YPG from a 400-sq-km area.

But he dismissed claims that the Kurdish YPG, which Ankara calls a terrorist group, had withdrawn to a Kurdish-controlled canton to the east of the Euphrates River. The YPG says it has done so and U.S. officials agree that is mostly the case.

“At the moment, they are saying the YPG has crossed,” Erdogan said. “We are saying no they didn’t. The proof depends on our own observation.”

Erdogan’s remarks appeared to be in reference to comments made by a U.S. defense official to Agence France Presse Monday that Kurdish forces had “all” moved east of the Euphrates.

Turkey launched the cross-border operation last week, saying it had a dual aim of driving away jihadists and ensuring Kurdish forces did not fill the void that was left by extending their control of territory along Turkey’s border.

Turkey is concerned that Syrian Kurdish fighters could embolden Kurdish militants waging an insurgency on its soil.

The United States has been alarmed by Turkey’s offensive against Kurdish forces, which Washington has backed in its bid to destroy ISIS. U.S. officials have urged Ankara to focus its attacks on the Islamist militants instead.

Turkish warplanes renewed air strikes on ISIS sites in northern Syria on Friday, CNN Turk reported, the latest attacks since Turkish-backed forces began the incursion on Aug. 24. Turkish officials have said Turkish-backed forces in recent days have struck westwards, in jihadist areas.

The YPG is part of a broader U.S.-backed coalition in Syria called the Syrian Democratic Forces. Washington has supported the group in its battle against ISIS but Ankara regards it as an extension of the PKK, the outlawed Kurdish militant group in Turkey.

“Nobody can expect us to allow a terror corridor on our southern border,” Erdogan said. Turkey, he added, had sought the establishment of a “safe zone” in Syria but the idea had not received the backing of other world powers.

On Thursday, Ankara said it had cleared dozens of villages of “terrorists” after taking the key border town of Jarablus without much resistance on the first day of the offensive.