There is "great fear" among residents and medical workers in Idlib as the threat of a large-scale military operation looms over Syrias last major opposition bastion, the provinces health chief said. "I fear we are about to face the most catastrophic crisis in our war," Munzer Al-Khalil told AFP in an interview in Geneva late Friday. The orthopedic surgeon, who heads the health directorate in the opposition region, said he had travelled to the Swiss city to urge diplomats and UN officials to do more to avert a "catastrophe". Seized from regime forces in 2015, Idlib and adjacent areas form the final major chunk of Syrian territory still under opposition control. It is home to some three million people -- around half of them displaced from other parts of the country, according to the United Nations. Damascus, which has retaken a succession of opposition bastions this year, has set its sights on Idlib, which is held by a complex array of factions and extremists. But a major military operation is expected to pose a humanitarian nightmare because there is no nearby opposition territory left in Syria where people could be evacuated. "There is a great fear of the advancement of the regime, since there is no other Idlib. There is no other place to go," said Khalil, who was due to travel back to Syria over the weekend. On Saturday, Russian air strikes against Idlib reached their "most violent" in a month, a monitor said. The spike in violence came after Russia, fellow regime ally Iran and opposition-backer Turkey on Friday failed to immediately agree on a solution to avert an imminent regime offensive. Khalil noted a hike in attacks on hospitals in Idlib -- there have been two in the past week alone -- warning this could mark the run-up to a full-scale assault. "When they decide to take an area, they first attack hospitals," he said. "I am worried this has already started." Bashar al-Assads regime has been repeatedly accused of using chemical weapons during the conflict, and Khalil said there was concern such weapons could be used in Idlib. "What we really fear are (conventional) attacks on vital, crowded locations like a market, a school, a hospital. When these are attacked the casualties are much higher." The UN has estimated that a military offensive against Idlib could force as many as 800,000 people to flee their homes, in what would be one of the largest displacements yet in Syrias seven-year war. Khalil said his main fear was that there would be a massive exodus towards Turkey with people finding themselves squeezed between advancing regime troops and a closed frontier. "I fear there will be people dying trying to cross the border," he said. Khalil said Idlibs health system was in shambles, with the few remaining doctors forced to work in damaged hospitals with dwindling supplies for surgeries and patients often forced to take expired medicines.