Syrian rebels backed by Turkey said on Monday that Moscow and Damascus were trying to wrest control of two major highways in their last enclave in the north-west of the country in a bid to shore up the sanction-hit economy.
The sixth day of the campaign by government forces saw heavy aerial attacks targeting the city of Jisr Al Shughour and the Al Ghab plain, as well as the towns of Al Latamenah and Maarat Al Numan in the south of Idlib province, the rebels said.
Taking those areas would bring President Bashar Al Assad close to regaining control over the strategic M5 and M4 highways from Aleppo to Hama and Latakia on the Mediterranean coast, two of Syria's most important pre-war arteries.
The first few days of the assault struck at towns in northern Hama and southern Idlib province inside a buffer zone agreed in September between Russia and Turkey as part of a deal which averted a major offensive on the area, the last major foothold of the Syrian rebellion.
Russia and the Syrian army say they are responding to stepped up attacks by hardline militants on government-held areas and deny indiscriminate strikes that medics and rescuers say have killed dozens of civilians in recent days, knocked down at least five medical centres, and paralysed day-to-day life.
The United Nations has said the attacks have included the worst use of barrel bombs by the Syrian army in 15 months. It said an estimated 323,000 people have been displaced in northwest Syria since September last year.
Residents said tens of thousands have fled their homes, many to camps on the Turkish border, since the latest offensive began. Some who failed to reach those camps sought shelter in olive orchards, residents and witnesses said.
"The shelling and aerial strikes have increased in intensity and ferocity and the area hit has widened with more intensity," said Naji Mustafa of the Turkey-backed National Liberation Front (NLF) rebel group.
Russia says Turkey has not done enough to evict militants from the buffer zone or to open the M5 and M4 highways that link cities held by the government and run from Syria's southern tip near the border with Jordan to the northern border with Turkey.
Opening the commercial and passenger routes through Idlib province would reassert the state's control over a fragmented economy that sprung up during eight years of conflict and now facing US and EU sanctions, economic experts say.
Russian President Vladimir Putin said last week that he did not rule out a full-scale assault on militants in Idlib province, after Russian officials publicly questioned how far they would continue to tolerate militant control.
The Syrian opposition seeking to topple Mr Assad accuse Moscow of using the jihadists as a pretext to step up attacks on civilian areas and put pressure on Turkey.
"There has long been a Russian aim to capture these highways. This is rejected as a princple of our revolution and it would have meant the displacement of tens of thousands of people who live in the area and refuse to be under Russian (rule)," said Major Yousef Hamoud, spokesman for the Turkey-backed National Army.
The loss of opposition control over the highways would mean the loss of a financial asset for the rebels, as well as be a sign of their weakening hold on their last enclave.
It would also undermine a sphere of influence that Turkey has carved out in recent years in Syria.