Turkey will launch a new military operation in northern Syria within days, targeting Kurdish militia fighters who are supported by U.S. troops east of the Euphrates river, President Tayyip Erdogan said on Wednesday.
Ankara and Washington have long been at odds over Syria, where the United States has backed the YPG Kurdish militia in the fight against Islamic State insurgents.
Turkey says the YPG is a terrorist organisation and an extension of the outlawed Kurdistan Workers Party (PKK), which has waged an insurgency against the state in southeastern Turkey for 34 years.
Turkey has already intervened to sweep YPG fighters from territory west of the Euphrates in military campaigns over the past two years, but up until now, it had not gone east of the river - partly to avoid direct confrontation with U.S. forces.
But Erdogan's patience with Washington over Syria - specifically a deal to clear the YPG from the town of Manbij, just west of the Euphrates - seems to have worn thin.
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"We will start the operation to clear the east of the Euphrates from separatist terrorists in a few days. Our target is never U.S. soldiers," Erdogan said at a speech at a defence industry summit in Ankara.
"This step will allow for the path to a political solution to be opened and for healthier cooperation."
Turkey has repeatedly voiced frustration about what it says are delays in the implementation of the Manbij deal, saying last month that the agreement should be fully carried out by the end of this year.
Turkish and U.S. troops began joint patrols near Manbij last month, but that cooperation has also been complicated as Turkey has shelled Kurdish fighters to the east of the Euphrates.
A U.S. official, speaking on the condition of anonymity, said Erdogan's comments had created concern in the Pentagon that the fight against Islamic State militants could be affected.
The official said there was concern that a Turkish operation could distract Kurdish YPG fighters away from the middle Euphrates River Valley.
In March, another Turkish offensive against the YPG affected the fight against Islamic State and led to an "operational pause" in eastern Syria. The Pentagon says it has about 2,000 troops in Syria.
Last month the United States said would establish observation posts on the border between Kurdish-held northern Syria and Turkey after Turkish cross-border shelling killed four Kurdish fighters.
The Kurdish-led Syrian Democratic Forces (SDF) said the Turkish attacks had led to a temporary halt in the U.S.-backed campaign the SDF are waging against Islamic State near the Syria-Iraq border.
Three observation posts have now been set up, a U.S. official told Reuters on Wednesday. The official said the positions were clearly marked and any force attacking them "would definitely know they are attacking the United States".
Erdogan's announcement came after Turkish officials held talks in Ankara this week with the U.S. special representative for Syria, Jim Jeffrey. An SDF military source said Jeffrey met the SDF leadership in northern Syria on Wednesday.
Erdogan said Turkey was the victim of a "stalling tactic" over Manbij and Islamic State no longer posed a threat in Syria. "Now, it's time to realise our decision to disperse the circles of terror east of the Euphrates. The fact that we have deep differences in perception with the United States is no secret," he said.
"A stalling tactic has been used in Manbij and is still being used... There is no threat named Daesh in Syria anymore. This is a fairytale," he said, using an Arabic acronym for the militant group.
The YPG still controls a large swathe of northeast Syria, on Turkey's southern border. Turkey regards the YPG as an extension of the PKK. More than 40,000 people, mostly Kurds, have died in the PKK's conflict with Ankara. Turkish authorities fear the conflict could be stoked by the YPG presence across the border.