Fighters shot down a Syrian regime warplane in the opposition stronghold of Idlib province on Wednesday, opposition sources and a war monitor said. A pilot who ejected from the plane was captured by the Tahrir al-Sham group, according to the Syrian Observatory for Human Rights, which reports on the war using a network of sources. Syrian state media made no initial mention of such an incident. An AFP correspondent saw smoke rising in the south of Idlib province above the debris of the plane, with its wing bearing the Syrian flag. The extremist Tahrir al-Sham, the most powerful group in the area, said its fighters had shot down a Sukhoi 22 jet that had taken off from a Syrian air base in Homs province. Tahrir al-Sham’s statement did not say how the plane had been shot down. The Observatory said heavy machine guns had been used. The jet was downed near Khan Sheikhoun, an opposition-held town that was hit by a sarin gas attack in 2017 and is now being targeted in a Russian-backed regime offensive. Regime forces advanced on Khan Sheikhoun on Wednesday, approaching to within a few kilometers (miles) of the town. An opposition commander told Reuters that the town, in opposition hands since 2014, was in “great danger”. Dozens of people were killed in Khan Sheikhoun in 2017 in the poison gas attack that prompted President Donald Trump to order a missile strike against the Syrian air base from where the United States said it had been launched. An investigation conducted by the United Nations and the Organization for the Prohibition of Chemical Weapons said the Syrian regime was responsible for releasing sarin on the town on April 4, 2017. Damascus denies using such weapons. Syrian opposition factions have shot down regime planes on several occasions during the war that spiraled out of the uprising against Bashar Assad in 2011. Also on Wednesday, Russian airstrikes killed four civilians near the town of Maaret Hurma in the south of Idlib province, said the Observatory. They included a paramedic, an ambulance driver, and a rescue worker. Clashes on Wednesday killed 16 regime forces, as well as 24 extremists and seven allied factions, the Observatory said. AFP correspondents have reported seeing dozens of families flee fighting over the past few days, heading north in trucks stacked high with belongings. The northwestern Idlib region is part of the last major stronghold of the opposition to Assad. Assad’s side had struggled to make any gains in the area in an offensive that got under way in late April. But since the collapse of a brief ceasefire this month, it has managed to take several significant positions, including the town of al-Habeet on Saturday. The advance toward Khan Sheikhoun threatens to encircle the last remaining pocket of opposition-held territory in neighboring Hama province, including the towns of Morek, Kafr Zeita and Latamneh. The humanitarian adviser to the UN Special Envoy for Syria said the new surge in violence in the northwest threatened the lives of millions after more than 500 civilians were killed since late April. The conflict has killed more than 370,000 people and displaced millions at home and abroad since starting with the brutal repression of anti-regime protests in 2011.