Amman: Russian and Syrian jets escalated strikes on rebel-held Idlib and Hama provinces, several days after extremist rebels opened an offensive against government-held parts of northwestern Syria, rebels and witnesses said on Sunday.
The bombing campaign in heavily populated civilian areas shattered six months of relative calm.
Russian-inspired ceasefires had given a temporary reprieve to tens of thousands of people living in rebel-held northwestern Syria.
But now thousands of civilians who had been returning to their homes have headed back to the relative safety of refugee camps along the Turkish border, which are protected under Russian-Turkish understandings, residents said.
"People are very afraid things have gone back to what they were and returned back to camps - there is no longer any hope," Ahmad Thaib, a resident of Jabal al Zawya.
The strikes were retaliation for last Tuesday's assault against Hama province, which were spearheaded by Tahrir Al Sham, the extremist Turkistan Islamic Party and rebels fighting under the banner of the Free Syrian Army (FSA).
The bombing campaign also comes shortly after a tripartite deal struck by Moscow, Ankara and Tehran to deploy an observer force in Idlib, a province where the former Al Qaeda's Syrian offshoot has cemented its control after it crushed opponents.
Western-vetted rebels said the Russian strikes targeted FSA groups who had signed the ceasefire agreements Russia had promoted, casting doubt on whether Moscow had any intention of shoring up the ceasefire.
Civil defence workers and residents said dozens of strikes struck the major towns of Khan Sheikhoun, Jisr Al Shaqour, Saraqeb and scores of villages.
At least five hospitals and several civil defence centres have been knocked out of action since the start of the counter offensive.
Rebel sources said over fifty fighters were killed on Saturday after jets believed to be Russian targeted a training camp run by Failaq Al Sham, an FSA rebel group, which is at odds with extremist groups.
Russia's defence ministry says it is attacking hard-line extremist militants.
It denies accusations it has targeted infrastructure and medical centres to force rebels into local truces that effectively restore President Bashar Al Assad's grip on the country.
On Friday, Russia's defence ministry said a submarine fired Kalibr cruise missiles at extremist militants, who it said had tried to trap a group of Russian military policemen .
In the city of Khan Sheikhoun in southern Idlib, jets believed to be Russian destroyed the town's major power plant on Friday, one witness said. The plant feeds northern Hama and southern Idlib.
On the outskirts of Jarjanaz, a strike barely missed a camp where hundreds of displaced people were sheltering, a resident said.
At least several people were killed near the town of Kafr Sajna, one of the sites where the Russian missiles landed, witnesses said.
A video downloaded by activists showed a rescue worker trying to extinguish the flames on a young body charred by an incendiary bomb.