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ANKARA (Reuters) - President Tayyip Erdogan said on Friday that Turkey needed to clear the Afrin region of northwest Syria of Kurdish YPG militia fighters, who have been in the forefront of the battle there against Islamic State insurgents.

Erdogan, speaking in Ankara to officials from the ruling AK Party, also expressed disappointment with the United States, which backs the YPG.

Ankara views the YPG as an extension of the outlawed Kurdistan Workers Party (PKK), which has fought a decades-long insurgency in Turkey and is designated a terrorist group by Turkey, the United States and European Union.

“We need to cleanse Afrin of the structure there called the YPG terrorist organization,” Erdogan said.

Turkey has long been angered at the United States’ support for the YPG and Erdogan has repeatedly accused Washington of violating the alliance between the two countries.

“We are greatly disappointed by the United States not keeping its promises. Many issues that we could have resolved easily...were pushed to a dead-end,” he said.

Turkey has expressed displeasure at the U.S. stance toward a deal between the YPG and Islamic State under which hundreds of militants withdrew from the Syrian city of Raqqa before it fell to the Syrian Democratic Forces.

Erdogan has previously said that Afrin would be on Ankara’s agenda after its current operation in Syria’s Idlib province, where Turkey and Russia have set up observation points under a “de-escalation” deal agreed by Ankara, Moscow and Tehran to ease the bloodshed in the six-year-old Syrian conflict.

Erdogan said considerable progress had been made in Turkey’s military operation in Idlib due to the co-operation of the three powers. Russia and Iran back Syrian President Bashar al-Assad and Turkey supports rebels opposing him, but all share a common foe in Islamic State.