Assad has now set his sights on Idlib, and his forces have stepped up bombardment of the densely populated province since the beginning of the month.
Turkey, which already hosts about 3.5 million Syrian refugees, built a four-metre wall along the frontier with Idlib in late 2016 to prevent any attacks against Turkey and stop the flow of new arrivals.
"A regime assault [in Idlib] would also create serious humanitarian and security risks for Turkey, the rest of Europe and beyond", he added. "We are not talking about a military operation".
A second rebel commander said: "They are getting new shipments of munitions - they don't need more than munitions".
"Terrorists must not be allowed to hold hostages indefinitely and use hundreds of thousands of civilians in Idlib as a human shield", he said, adding that "it is impossible to exist side by side with terrorists."
Turkish officials could not immediately be reached for comment.
Idlib is estimated to be now home to about three million civilians, including one million children, many of whom have already fled fighting elsewhere in Syria, or have been evacuated there under resettlement deals, as well as tens of thousands of fighters. Turkey has bolstered the Free Syrian Army there with troops and military observation posts that were part of an earlier cease-fire agreement put in place past year.
The area is Syria's last major stronghold of active opposition to the rule of President Bashar al-Assad. Representatives of Russia, Turkey and Iran, meet with the UN Special Envoy of the Secretary-General for Syria to discuss the situation in Syria.
The most important thing to do now is to separate the moderate opposition from the hardline jihadists, primarily from the local Al-Qaeda affiliate, Jabhat al-Nusra (now rebranded as Tahrir al-Sham). "Idlib province is... a sort of zone of responsibility of Turkey; it is their responsibility to separate the moderate opposition from the extremists", Lavrentiev said.
Turkey, which has sent troops to Idlib and supports some of the armed opposition, called for "an immediate ceasefire" and urged "the global community to vocally and actively support our efforts to this end". The Turkish army has also sent troops into Syrian rebel-held territory further east, in an area north of Aleppo city.
A Kurdish-Arab alliance launched an assault on the pocket's main town of Hajjin on Monday, with support from the US-led coalition fighting Daesh.
In another show of force, the Pentagon launched a surprise exercise in southern Syria on Friday after Russian Federation threatened military action in an area of Syria where USA troops are located.
"The consequences of inaction are enormous".
"Predications" made by Western media and politicians of a disaster in Idlib are unacceptable, given all the efforts of Moscow and its partners, and earlier examples of reconciliation in Syria, a top Russian diplomat says.
HTS is considered a terrorist group by Russian Federation and Turkey, with Russian Federation and the Syrian government using the group's presence in the area as one of the main justifications for its assault.