The Assad regime shelled Syria's northwestern Idlib province on Wednesday after Russia resumed air strikes the previous day.
Syrian President Bashar al-Assad has sworn to recapture every inch of Syria and has made big gains against the opposition since Russia joined his war effort in 2015.
Last week, a source close to Damascus said the government was preparing a phased offensive to recover Idlib province, but Turkey, whose army has a string of observation posts around the edge of the opposition area, has warned against such an assault.
Russian air raids ceased in and around Idlib on August 15, but pro-Syrian government forces have maintained an aerial bombardment and shelling.