Syrian regime forces edged forward in the northern province of Aleppo with air cover from Russian warplanes, but faced fierce resistance from rebel forces in the country ' s center. Since Moscow began its air campaign in support of its Damascus ally on September 30, the army and its allies have launched four ground offensives against rebel forces in northern and central Syria, the latest push forward in a years-long conflict that a monitor said has killed more than 250,000 people. Syrian troops have gone on the attack in Aleppo, Hama, Homs and Latakia provinces, taking advantage of Russian air strikes against Al-Qaeda affiliate Al-Nusra Front and other rebel groups. Control of Aleppo city -- Syria's pre-war economic hub -- has been divided since mid-2012, with government forces controlling the western part of the city while much of the surrounding province is held by rebel groups, ranging from Nusra and others in the west to the Islamic State group (IS) in the east. The front lines there and in the surrounding countryside have long been static. On Saturday, three senior Nusra members, one of them a US-designated "global terrorist", were killed in an air strike in Aleppo province on Thursday, a monitoring group said. And regime troops seized at least five villages and several strategic hilltops on Saturday, bringing them to the edges of Al-Hader, just south of Aleppo city, the Britain-based Syrian Observatory for Human Rights said.