BEIRUT - Russian-backed Syrian troops have encircled a Turkish observation post as they approach the city of Maarat al-Numan in an offensive into the last significant rebel pocket of Syria, sources on both sides said on Tuesday.It is the first major bout of fighting since the leaders of Turkey, Russia and Iran agreed in Ankara in September to "de-escalate" conflict in the northwestern province of Idlib after a months-long campaign that forced at least 500,000 civilians to flee. With diplomacy on a Syria peace settlement stalled, the de-escalation deal has been unraveling of late. Insurgents controlling the Idlib region, which borders Turkey, include the powerful Islamist militant group Tahrir al-Sham as well as Turkish-backed rebel factions.The sources said the Turkish military outpost, near the village of Surman and the main rebel-held highway that extends to the northern, government-controlled city of Aleppo, and eventually the capital Damascus further south, was now surrounded by Syrian forces, aided by Iranian-backed militias.Turkey has 12 outposts in Idlib under a security arrangement struck with Russia and Iran, Syrian President Bashar al-Assad's major allies, in 2017. There have been several shooting attacks on the outposts since then, prompting Turkish retaliation, but Russian pressure on Damascus halted firing by Assad's forces.FRESH HUMANITARIAN CRISISAs government forces have thrust into the southeast of Idlib over the past two weeks, thousands of people have fled Maarat al-Numan, a major city that has been a refuge for families who fled other areas of Syria recaptured by Damascus earlier in an 8-1/2-year-old civil war.A Russian-led aerial bombing campaign has helped Syrian government forces close in on the strategic city, a major factor in the flight of civilians further north closer to the border with Turkey.The offensive has killed scores of civilians and sparked so far an exodus of at least 80,000 people in all of Idlib province, including 30,000 in the past week alone, the United Nations said on Monday.Eight people, including five children, died when bombs dropped by Russian jets struck displaced families as they were seeking shelter in the village of Jubas, east of the town of Saraqeb, rescuers and area residents said."The bombing killed two families who had set up tents," said Mohammad Rasheed, a local rights activist.At least 12 other people were killed this week when Russian and Syrian air force jets hit civilian convoys on main roads where families were fleeing near the Turkish frontier, according to humanitarian groups and witnesses.U.N. Secretary-General Antonio Guterres said on Monday he was "alarmed by the scale of the military operation and reported attacks on evacuation routes" used by civilians fleeing north towards the Turkish border."The Secretary-General reminds all parties of their obligations to protect civilians and ensure freedom of movement," Guterres's spokesman said, urging an immediate cessation of hostilities.Moscow and Damascus both deny allegations of indiscriminate bombing of civilian areas and say they are fighting al Qaeda-inspired Islamist militants.Syrian government forces are now some six km (four miles) east of Maarat al-Numan, according to Hajj Abu Rasheed from the National Liberation Front, a rebel coalition backed by Turkey.He said a suicide bomber blew himself up at an army outpost in Jerganaz that had been captured on Monday by Iranian-backed militias and government troops in their advance.