Both Russian and Turkish President are due to meet on Wednesday to discuss the violence in northwestern Syria
Turkey sent reinforcements to the last rebel-held enclave in northwestern Syria as Russia stepped up air strikes there on Sunday, several sources said, three days before leaders Vladimir Putin and Tayyip Erdogan are due to meet for talks.
Residents as well as opposition and military sources said that Russian jets bombed villages around the city of Afrin on Sunday, intensifying air strikes on towns and villages held by Turkish-backed rebels which have escalated in the last week.
Rebel sources said at least five fighters from a Turkey-backed faction were killed while at least 12 civilians were injured when Russian jets flying at high altitudes dropped ordnance, according to a network of plane spotters who document sightings of jet fighters.
The Russian bombing campaign has spread from Jabal al Zawya in Idlib province further northwest to the Afrin area near the Turkish border, with shelling from Syrian army outposts aided by Iranian-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," Major Youssef Hamoud, the spokesman for the National Army, the main Turkish-backed rebel force, told Reuters.
The escalation comes days before Turkish President Erdogan is expected to meet Russian President Putin in Sochi on Wednesday to discuss a deal reached last year that ended a Russian-led Syrian army offensive that displaced nearly a million people in Idlib, in the largest humanitarian displacement of the over decade conflict.
Russian Foreign Minister Sergey Lavrov said in New York on Saturday progress in implementing the deal was slow. He also said the summit would discuss Ankara's obligation to end militant presence which Moscow blames for the violence, Tass News agency said.
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. Over four million people live in the densely populated opposition held northwest, including half a million in makeshift tents along the Turkish border who were driven by successive Russian led campaigns that regained territory seized by rebels.
Signalling its determination to deter any renewed assault to retake the bastion, Turkey has in the last two weeks beefed up dozens of bases where thousands of troops are stationed, military commanders coordinating 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," Hamoud added.
Military sources said a large Turkish military convoy with rocket launchers and tanks crossed into Syria overnight and was seen in the direction of Jabal al Zawya where Turkish bases are spread.
Turkey’s military conducted several operations since 2016 that have altered the course of the Syrian conflict in the north. Ankara has not given a figure on its troop deployment, by far the largest outside the Nato member state.
Already hosting 3.6 million Syrian refugees, Ankara is determined to prevent any further influx of migrants from Syria.