Turkey on Tuesday lashed out at what it called "unacceptable" remarks by a senior U.S. official suggesting Ankara had encouraged the rise of Al-Qaeda-linked groups in Syria's Idlib province.
Brett McGurk, the senior U.S. envoy to the international coalition fighting Daesh (ISIS), said Idlib province had become "the largest Al-Qaeda safe haven."
In comments to the Washington-based think tank the Middle East Institute last week, McGurk criticised some U.S. partners for sending in "tens of thousands of tons of weapons" to Syria. He did not name which partners.
He added that the way those foreign fighters come in to Syria "may not have been the best approach" because Al-Qaeda had taken full advantage of it.
"And Idlib now is a huge problem. It is an Al-Qaeda safe haven right on the border of Turkey. So that's something, obviously, we will be in very close discussions with the Turks on."
But Turkish presidential spokesman Ibrahim Kalin said it was not Turkey which controlled Idlib province.
"Attempting to associate Turkey with that terror organization in Idlib, making such an implication is unacceptable," Kalin told the TvNet broadcaster.
"Why? Because we are not controlling Idlib."
Idlib is the only province in Syria that remains entirely beyond regime control after having been captured in 2015 by an alliance of extremists and rebels.
Kalin said McGurk's comment could not be considered to have been made with a "good intention."
Foreign ministry spokesman Huseyin Muftuoglu added that Ankara had protested against the "provocative" comments, without elaborating.
McGurk has been a frequent target of the ire of the Turkish government for comments supporting Kurdish groups labelled as terror organisations by Ankara.
Turkish Foreign Minister Mevlut Cavusoglu has called for McGurk to be fired, claiming he was backing Kurdish militants in Syria.
In June, McGurk had talks with Turkish officials in Ankara.
The alliance that captured the Idlib province was dominated by Al-Qaeda affiliate Nusra Front, and the rebel Ahrar al-Sham, a key Islamist faction backed by neighbouring Turkey and Gulf states.
In the years since the capture, Nusra ostensibly split with Al-Qaeda, renaming itself Jabhat Fatah al-Sham and becoming the backbone of the so-called Hayat Tahrir al-Sham (HTS) coalition that last month captured Idlib city.
Turkey's NATO allies in the early years of the civil war accused Ankara of turning a blind eye - or even aiding - the rise of Islamist groups in Syria including Daesh militants. But Turkey always angrily rejected the charges.