Turkey said on Friday that a huge banner of jailed Kurdish militant leader Abdullah Ocalan unfurled in central Raqqa by US-backed Kurdish forces would further harm already fraught relations between Ankara and Washington.
The banner of Ocalan was raised on Thursday at a ceremony to mark Raqqa's capture from Islamic State in a campaign spearheaded by Kurdish Syrian YPG fighters, with military support from the United States.
Turkey considers the YPG to be an extension of Ocalan's Kurdistan Worker's Party (PKK), which has waged a three-decade insurgency in Turkey's southeast and is designated a terrorist organization by the United States, European Union and Turkey.
Ankara says that weapons supplied to fighters in Syria have ended up in PKK hands, threatening Turkish security.
"I wonder what more evidence the US needs to accept that the ... YPG is a terrorist organization," Turkish Prime Minister Binali Yildirim told reporters as he left a mosque in Istanbul.
"Displaying a photograph of the PKK terrorist leader damages US-Turkey relations very seriously. With this move, the US is not only cooperating with terrorists, but they are endangering the future of Syria."
The banner of Ocalan was raised on Thursday by an all-female Kurdish militia. Kurdish YPG commanders and fighters were also shown chanting "Long Live Apo!", as Ocalan is known by his followers, in a video of celebrations distributed by the YPG press office.
President Tayyip Erdogan echoed Yildirim's condemnation.
"How can the US explain the poster of Ocalan in Raqqa? Is this the way they are cooperating with us in the struggle against terror?" he said. "You are not standing by us against terrorism. You wouldn't allow this if you were."
Ocalan has been in jail in Turkey since 1999 on a treason conviction. He negotiated a truce from his prison cell, but the ceasefire broke down two years ago and thousands have died in mainly Kurdish southeast Turkey in renewed violence since then.
The banner incident comes amid a diplomatic spat between Ankara and Washington over the detention of a Turkish employee of the US consulate in Istanbul that has seen the two countries suspend visa services for each other's citizens.