The United States’ support for the Syrian Kurdish YPG militia is a “big mistake”, Turkish Foreign Minister Mevlut Cavusoglu said late on Saturday, adding that the issue had strained ties between the NATO allies.

Turkey has been infuriated with Washington’s support for the YPG, which it views as an extension of the outlawed Kurdistan Workers Party (PKK) waging a decades-long insurgency on Turkish soil.

U.S.-Turkey ties have been strained over issues including U.S. policy in Syria, the case of an American pastor in Turkey, and Turkey’s demands for the extradition of U.S.-based cleric Fethullah Gulen, whom Ankara blames for a 2016 failed coup.

Cavusoglu, who is in the United States on an official visit, said tensions between Ankara and Washington stemmed from U.S. support for the YPG and the issue of Gulen, against whom he said the FBI had launched an investigation.

“Despite knowing and acknowledging that (the YPG) is the same organisation (as the PKK), seeing this cooperation as necessary is really a big mistake,” Cavusoglu said, adding that he would discuss bilateral relations with his U.S. counterpart Mike Pompeo on Tuesday.

On Sunday, Turkish Defence Minister Hulusi Akar said he had told U.S. Chief of Staff Joseph Dunford that Turkey expected the United States to stop its support for the YPG as soon as possible, according to the state-owned Anadolu news agency.

“We reiterated our warnings and stated that we expected our U.S. counterparts to take the necessary measures and end their relationship with the YPG, which is no different than the PKK, as soon as possible,” Akar was quoted as saying.

“We reminded them that the United States, our ally and strategic partner here (Syria), and U.S. soldiers cooperating with such an organization (YPG) cannot be acceptable in any way,” he said.

Tensions between the NATO allies have eased slightly in the last month following pastor Andrew Brunson’s release and the beginning of joint patrols in Syria’s Manbij as part of a roadmap agreed by the two countries in June.

The two countries last month also lifted mutual sanctions against top officials, imposed in response to Brunson’s detention and arrest.

Earlier this month, Washington pledged millions of dollars to help capture three top PKK militants in a move that Turkey welcomed, but said was late and insufficient.

Since the attempted putsch, Turkey has jailed 77,000 people as they face trial, suspending or dismissing some 150,000 civil servants and military personnel over alleged links to Gulen.

“On both issues, we are not only a hundred percent but a thousand percent right,” Cavusoglu said.