A Syrian regime helicopter was shot down on Tuesday in the province of Idlib as a military offensive continued, with strikes killing at least 12 civilians in the country's last rebel pocket.
The Syrian Observatory for Human Rights said the aircraft crashed near the village of Qaminas, southeast of Idlib city, after being hit by a rocket fired by Turkish forces.
Ankara, which supports Syrian armed groups in the rebel enclave in north-west Syria, acknowledged the attack but did not claim responsibility.
Half of those killed in the strikes were minors, the Britain-based Syrian Observatory for Human Rights said.
Government forces seized control of a highway in north-west Syria for the first time since 2012 on Tuesday as they pressed their campaign to eradicate the last rebel strongholds in Idlib province and the Aleppo countryside.
Russian President Vladimir Putin will discuss the situation in Syria with Turkish President Tayyip Erdogan by phone later on Tuesday amid rising tensions over the Syrian province of Idlib, the Tass news agency cited the Kremlin as saying.
Moscow said earlier on Tuesday that all attacks on Russian and Syrian government forces in Idlib should stop and that agreements it had struck with Turkey on the conflict there had to be upheld.
A wave of displacement that has seen around 700,000 people flee a regime offensive in Syria's Idlib region is the biggest of the nine-year-old conflict, the United Nations said Tuesday.
"In just 10 weeks, since 1 December, some 690,000 people have been displaced from their homes in Idlib and surrounding areas," a spokesman for the Office for the Coordination of Humanitarian Affairs said.
"This is, from our initial analysis, the largest number of people displaced in a single period since the Syrian crisis began almost nine years ago," David Swanson said.
The conflict in Syria has killed more than 380,000 people since it erupted following the brutal repression of popular demonstrations demanding regime change.
The battle for Idlib is a crucial stage of a war that has killed hundreds of thousands of combatants and civilians, made millions refugees in their own country or overseas, and fractured the wider Middle East since it broke out amid the Arab uprisings in 2011.
Forces arrayed against Mr Al Assad, Syria's ruler for nearly 20 years, have failed to dislodge him but he now presides over a devastated country. Moscow's military intervention in 2015 helped swing the war decisively in Mr Al Assad's favour.
Despite being on opposing sides, Turkey and Russia collaborate for a political solution to the conflict.