Syrian forces gained more ground from insurgents in the country’s northwest on Thursday, edging closer to a major rebel-held town a day after militants shot down a government warplane in the area. The government offensive, which intensified last week, has displaced nearly 100,000 people over the past four days, according to the Syrian Response Coordination Group, a relief group active in northwestern Syria. Syrian troops have been on the offensive in Idlib and its surroundings, the last major rebel stronghold in Syria, since April 30.