The intelligence chiefs of Turkey and Syria held a rare meeting in Moscow, a senior Turkish official confirmed to dpa on Tuesday.
While there has long been speculation of high-level contact between Ankara and Damascus, who are on opposing sides in the conflict, such a meeting was publicly confirmed for the first time, a Syrian source said.
They also "talked about the possibility of working together against the YPG in north-eastern Syria," the Turkish source said, referring to the Syrian Kurdish People's Protection Units.
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Turkey launched an incursion into north-eastern Syria in October after the U.S. pulled back its troops and abandoned its Syrian Kurdish partners.
According to Syrian state news agency SANA, Mamlouk called for an immediate and full withdrawal of Turkish troops from Syria.
While Turkey has backed rebels seeking to oust al-Assad, Damascus has been propped up by Russia.
Recep Tayyip Erdogan has previously called al-Assad a "murderer," and the Syrian leader has branded the Turkish president a "thief."
Erdogan said Tuesday that Turkey could resume its operations in Syria at any minute "to prevent the regime's attempts to violate the ceasefire."
Referring to truces brokered with Moscow and Washington, he cited "serious differences" between what was agreed and ground realities.
"Everyone should see and accept that it is no joke anymore and that Turkey will definitely do it when it says, 'We will do it'," he warned.