Video distributed by the Turkish army showed what it said was the convoy starting to move on Thursday night, with military vehicles traveling along a road in darkness.
Turkey says its operation, along with Syrian rebel groups it backs, is part of a deal it reached last month with Russia and Iran in Astana, Kazakhstan, to reduce fighting between insurgents and the Syrian government.
The army said its forces in Syria were conducting operations in line with rules of engagement agreed with Russia and Iran.
However, the deployment is also intended to rein in the Kurdish YPG militia, which holds the adjacent Afrin region, a senior rebel official involved in the operation said.
Broadcaster CNN Turk reported on its website that there was a clash in Idlib countryside near the Ogulpinar border post in Turkey’s Reyhanli district.
It said the sound of “doshka” (machine-gun) fire from across the border could be heard in Reyhanli district and it was not clear which forces were clashing.
The convoy was heading toward Sheikh Barakat, a high area overlooking rebel-held territory and the Kurdish YPG-controlled canton of Afrin, the witnesses said.
President Tayyip Erdogan announced the deployment on Saturday, saying Turkey was conducting a “serious operation” with rebel groups it supports.