Rebels backed by Turkey are carrying out a new operation in Syria's north-western Idlib province, Turkish President Recep Tayyip Erdogan says.
The province is dominated by jihadist groups and Mr Erdogan has said he will not allow a "terror corridor" on the Turkish border.
The operation is led by Turkish-supported Free Syrian Army rebels.
Mr Erdogan said Turkish troops were not yet inside Idlib. Russia is providing air support.
Turkey, Russia and Iran agreed earlier this year to create a series of "de-escalation" zones in Syria, including Idlib, as part of moves to end the Syrian civil war.
The agreement came despite Russia and Iran backing President Assad and Turkey wanting him gone.
Idlib is mainly controlled by the jihadist group Hayat Tahrir al-Sham (HTS), an alliance led by a former branch of al-Qaeda, and has seen a big influx of civilians from other war-torn regions.
HTS wants to continue fighting the government despite the establishment of "de-escalation zones".
Mr Erdogan, in a televised speech, said he was not going to let civilians down.
"We are taking new steps to ensure security in Idlib," he said. "Today a very serious operation is ongoing in Idlib and this will continue."
"We will absolutely not allow the creation of a terror corridor along our borders," he added.
Though Mr Erdogan said Turkish troops were not yet in Idlib, he said last month that they would be deployed and residents and rebels believe an incursion is imminent.
Reports suggest a large military deployment on Turkey's southern border and residents near the Bab al-Hawa crossing told Reuters that a section of frontier wall had been removed.
Turkey conducted operation "Euphrates Shield" in neighbouring Aleppo province earlier this year against so-called Islamic States and Kurdish militia.