Turkey sent hundreds of military vehicles on Saturday into Syria's northwestern Idlib province, witnesses said, after Syrian government forces took control of a strategic town close to the provincial capital.
The fresh Turkish deployment came as officials from Turkey and Russia, who support opposing sides in Syria's nearly nine-year war, met in Ankara to discuss the fighting in the last major enclave of opposition to President Bashar al-Assad.
The escalation in Idlib has displaced more than half a million people and disrupted fragile cooperation between Russia and Turkey, which already hosts 3.6 million Syrians and fears another wave of refugees fleeing the latest offensive.
Turkish President Tayyip Erdogan, who backs some of the rebels who once aimed to topple Assad, threatened this week to repel the Russian-backed Syrian forces unless they withdraw from the region by the end of the month.
Despite the build-up of Turkish troops, Syrian government forces have pressed their advances, surrounding several observation posts that Ankara set up around a "de-escalation zone" agreed with Assad's backers Moscow and Tehran in 2017.
On Monday, eight Turkish military personnel were killed in shelling by Syrian government forces.
"Our check-points in Idlib continue their duties as usual and are capable of protecting themselves," Turkey's Defence Ministry said, adding they would respond to any new attack "in the harshest manner in accordance with legitimate defence".
The Syrian Observatory for Human Rights, a UK-based war monitor, said on Saturday that 430 Turkish military vehicles had crossed into Idlib in the last 24 hours.
Turkish forces were setting up a new post at Al-Mastoumah, on the southern approach to Idlib city, the Observatory said.
Syrian state TV broadcast live on Saturday from the strategic town of Saraqeb, located at the junction of the two main highways in Idlib that Assad seeks full control of, and lies less than 10 miles (15 km) southeast of Idlib city.
It said the army had taken full control over the town.