Turkey's offensive in Afrin against the Kurdish YPG militia, which began last month, has further complicated the multi-sided conflict in Syria, which will soon enter its eight year.
The YPG has been an important U.S. ally in the war against Islamic State militants, but Ankara sees it as an extension of the Kurdistan Workers Party (PKK), which is listed as a terrorist group by Turkey, the European Union and Washington.
Turkish President Tayyip Erdogan ramped up his verbal assault on the U.S. role in Syria on Tuesday, saying U.S. forces should leave Manbij, a Syrian city held by YPG-allied forces with support from the U.S.-led coalition.
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He said the United States was working against the interests of Turkey, Iran and maybe Russia in northern Syria.
"If the United States says they are sending 5,000 trucks and 2,000 cargo planes of weapons for the fight against Daesh (Islamic State), we don't believe this," Erdogan told members of his AK Party in parliament.
"It means you have calculations against Turkey and Iran, and maybe Russia," he said, repeating a call for U.S. troops to withdraw from Manbij.
In agreement with Iran and Russia, the Turkish military is setting up observation posts in parts of Idlib and Aleppo province. But tensions have flared as Turkish forces moved to set up one such observation position south of Aleppo.
The Turkish military said a rocket and mortar attack by militants killed one Turkish soldier while the observation post was being set up on Monday.
It was the second attack in a week on Turkish soldiers trying to establish the position near the front line between rebels and pro-Syrian government forces.
In an apparent warning to Ankara, a commander in the military alliance supporting Assad said the Syrian army had deployed new air defences and anti-aircraft missiles to front lines with rebels in the Aleppo and Idlib areas.
"They cover the air space of the Syrian north," the commander told Reuters. That would include the Afrin area where Turkish warplanes have been supporting the ground offensive by the Turkish army and allied Free Syrian Army factions.
Turkey, which supported rebel fighters trying to overthrow Assad, has worked with Assad's main international backers Russia and Iran in recent months to try to wind down the conflict in Syria. But all three countries remain deeply involved in the fighting, and stark divisions persist.
Iran urged Turkey on Monday to stop its two-week-old military offensive against the Kurdish YPG in northern Syria's Afrin region, which abuts Idlib.