Turkey is reportedly reinforcing its military observation posts inside rebel-held north-western Syria ahead of an expected government offensive.
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 posted a video online showing what they said were Turkish armoured vehicles and a lorry transporting a tank moving along a road leading from Kfar Lusin.
There was no official confirmation from the Turkish military, but a senior Turkish security source told Reuters news agency on Wednesday that the observation posts were being reinforced.
"We have a military presence there and if that military presence is damaged or attacked in any way, it would be considered an attack on Turkey and would therefore receive the necessary retaliation," the source said.
Why are there observation posts in Idlib?
Turkey established the observation posts to monitor a "de-escalation" agreement with Russia and Iran, which was intended to reduce violence along the frontlines.
The agreement also covered three other conflict zones in Syria that were recaptured earlier this year by the Syrian army with the support of Russian air strikes and Iranian-backed militias.
The Syrian government has said it now intends to "liberate" Idlib from Hayat Tahrir al-Sham (HTS), an al-Qaeda-linked jihadist alliance whose estimated 10,000 fighters control large parts of the province.
Foreign Minister Walid Muallem has said it hopes to avoid civilian casualties and regain territory through "reconciliation agreements", but that it is determined to defeat HTS "no matter the sacrifices".
The UN and Turkey agree that HTS - which they have both designated as a terrorist organisation - must be defeated, but not at the expense of thousands of civilian lives.
On Monday, Turkish President Recep Tayyip Erdogan called for a ceasefire and for a "comprehensive international terrorism operation" involving the "moderate" rebels his country supports.
He also warned that Turkey, which is already hosting 3.5 million Syrian refugees, would not be able to absorb the 800,000 people whom the UN believes would be displaced by an all-out assault on Idlib.
A senior UN aid official, Panos Moumtzis, on Thursday quoted a Russian official as telling a humanitarian taskforce in Geneva that "every effort to find a peaceful solution to the problem is being made".