Turkey has sent reinforcements to Afrin – north-western Syria’s last rebel-held district – as Russian air strikes on the area were stepped up.
President Vladimir Putin of Russia and his Turkish counterpart, Recep Tayyip Erdogan, are due to meet on Wednesday.
Russian jets bombed villages in Afrin on Sunday, residents as well as opposition and military sources said.
Air strikes on towns and villages held by Turkish-backed rebels have escalated in the past week.
Rebel sources said at least five fighters from a Turkey-backed faction were killed and at least 12 civilians were injured when Russian jets flying at high altitudes dropped ordnance, a network of plane spotters who document jet fighter sightings said.
The Russian bombing campaign has spread from Jabal Al Zawya in Idlib province farther north-west to Afrin, near the Turkish border, with shelling from Syrian army outposts supported by Iran-backed militias.
“Russia’s escalation has intensified this week and begun from Idlib province and now stretches to areas in northern Aleppo province along the border,” Maj Youssef Hamoud told Reuters. Maj Hamoud is the spokesman for the National Army, the main Turkish-backed rebel force.
Mr Erdogan and Mr Putin are expected to discuss a deal reached last year that ended a Russian-led Syrian army offensive on Idlib.
It displaced about a million people, the largest displacement of the conflict.
On Saturday, at the UN General Assembly in New York, Russian Foreign Minister Sergey Lavrov said progress in implementing the deal was slow.
He told Russia’s Tass news agency that Ankara’s obligation to end the presence of rebels – on whom Moscow blames the violence – would be discussed at the summit.
The ceasefire reached by Moscow and Ankara cemented significant gains by the Syrian army and Iranian-backed militias, but a major deployment of Turkish troops inside Idlib province halted further advances by the Russian-led offensive.
Convoys ‘poured in’
More than four million people live in Syria’s densely populated opposition-held north-west.
Half a million of them live in makeshift tents along the Turkish border, where they were driven by successive Russian-led campaigns to regain rebel-held territory.
In the past two weeks, Turkey has beefed up dozens of bases on which thousands of troops are stationed, military commanders co-ordinating with the Turkish army said.
“The Turkish army deployment is taking a combat posture with all the military bases reinforced and has poured in convoys whether of armoured vehicles, fighters or equipment,” Maj Hamoud said.
Military sources said a large Turkish military convoy with rocket launchers and tanks had crossed into Syria overnight and was seen in the direction of Jabal Al Zawya, where Turkey has bases.
Turkey’s military has conducted several operations since 2016 that have altered the course of the Syrian conflict in the north.