Activists saw a convoy heading towards some of the 12 posts set up last year under a “de-escalation” deal with the government’s allies, Russia and Iran.
Turkey supports rebel factions trying to overthrow President Bashar al-Assad.
Meanwhile, Turkish Defence Minister Hulusi Akar warned that an offensive would lead to a “humanitarian tragedy”.
“Bombing the region will not only cause damage to the civilians and migration but also lead to radicalisation,” Mr Akar was quoted as saying by Turkey’s state-run Anadolu news agency.
A sharp increase in hostilities and fears of further escalation have led to the displacement of more than 38,500 people since the start of September, according to the UN.
An estimated 2.9 million people, including one million children, are living in parts of Idlib, Hama and Aleppo provinces that are controlled by Turkish-backed rebels and jihadists.
What forces has Turkey deployed?
The Syrian Observatory for Human Rights, a UK-based monitoring group, said a Turkish convoy entered Idlib via the Kfar Lusin crossing on Thursday.
The convoy then split in two, it added, with one part heading towards the northern Hama countryside and the other going towards the countryside near the central Idlib town of Maarat al-Numan.
Activists from the pro-opposition Edlib Media Center also…