Turkish President Recep Tayyip Erdogan's spokesman accused the US military on Thursday of legitimizing a terrorist group after it re-tweeted a statement from a Kurdish-Arab alliance fighting against the self-styled "Islamic State."

The US backs the Syrian Democratic Forces (SDF), a multi-ethnic force dominated by Kurdish fighters battling IS in northern Syria.

The SDF is led by the powerful YPG militia, which Ankara considers to be the same as the Kurdistan Workers' Party (PKK) waging a more than three-decades-long insurgency in Turkey.  The US, EU and Turkey classify the PKK as a terrorist organization, but the West has diverged with Turkey over the nature of the Syrian Kurds' ties to the PKK.

"SDF confirms that it has no affiliation or ties to PKK," US Central Command, whose area of responsibility includes the Middle East, said on Twitter, re-tweeting a SDF statement in which the group said it would continue to focus on fighting IS and wanted good relations with all neighboring countries, including Turkey.

"Is this a joke or @CENTCOM has lost its senses? Do you believe anyone will buy this? The US must stop trying to legitimize a terrorist group," responded Ibrahim Kalin, an advisor and spokesmen for the Turkish presidency.

Defense Minister Fikri Isik said at a conference of ambassadors in Ankara on Thursday that he hoped the incoming Donald Trump administration would "correct the mistake" of the US allying with the YPG against IS. 

US support for the SDF has been a major sore spot in relations with Turkey, a member of the US-led coalition against IS. 

The Turkish military and Syrian rebels it backs intervened in northern Syria last August to clear IS from a section of the border and prevent Kurdish forces from controlling nearly all of the border. There have been occasional clashes with the Kurdish-Arab alliance.

Erdogan has threatened to move on SDF-controlled Manbij after the Turkish military captures the IS-stronghold of al-Bab. But rebels and the Turkish military have been bogged down for nearly two months fighting entrenched IS forces in al-Bab. Erdogan has also suggested the Turkish military will advance on IS' self-declared capital Raqqa.

A Turkish move on Manbij threatens to lead to clashes between the US-backed SDF and a NATO ally, undermining international efforts to fight IS and further complicating an already combustible situation.

The SDF is considered the best fighting force against IS and has drastically reduced territory controlled by the terror outfit in northern Syria. Since launching an offensive to retake Raqqa in December, SDF forces have advanced to within about 30 kilometers (20 miles) of Raqqa.

The United States wants Arab contingents within the SDF to hold Raqqa if and when it falls. A large Kurdish military presence would undermine civilian support in the Arab town, US officials have said.

The continued war of words between the US and Turkey over the Syrian Kurds comes as Ankara expands cooperation with Russia to reach a political solution in Syria. Ankara has blocked any participation of the Syrian Kurds at Turkish and Russian-led political talks between the opposition and Syrian government scheduled in Kazakhstan on January 23.