"Russian warplanes resumed bombing Idlib province after a 22-day pause," said SOHR chief Rami Abdel Rahman.
The White House had warned that a "reckless attack" on one of the last rebel-held areas in the country would be a "grave humanitarian mistake" that could destabilize the peace process and kill thousands. Top U.S. generals also cautioned that without more precise planning, Russia was risking the deaths of an untold number of civilians.
Iran, which supports Syrian President Bashar al-Assad, said it would also be helping remove militants from Idlib with the least amount of casualties possible.
Moscow responded by accusing the administration of President Donald Trump of lacking "a comprehensive approach" to the Syrian conflict and called Idlib "a hornets' nest of terrorists."
In light of the strikes, Turkey announced that it was moving tanks to its border with Syria and reinforcing its observation posts near Idlib.
The SOHR and a rebel source told Reuters that the warplanes bombarded countryside around Jisr al-Shughour on the western edge of the rebel enclave of Idlib after weeks of lull, killing 13 civilians but no fighters.
The White Helmets also report civilian casualties as a result of the airstrikes:
Five children were killed, and more than 12 others are injured in a new massacre unleashed by Russian aircraft in the city of #JisralShughur in #Idlib western countryside. #WhiteHelmets teams mobilized quickly to retrieve the dead and transfer the wounded to medical points. pic.twitter.com/5DD2tsFZRj— The White Helmets (@SyriaCivilDef) 4 вересня 2018 р.
Three women were killed, and many other injured, as a result of the renewed aerial bombardment on #Muhambel city in #Idlib western countryside. #WhiteHelmets teams rushed to the scene and worked to retrieve the dead and assist the wounded, evacuating them to the medical point. pic.twitter.com/9kyZSPViJi— The White Helmets (@SyriaCivilDef) 4 вересня 2018 р.
5 Civilians were Injured, and mass destruction of civilian property following an aerial bombardment of the popular market in #Mohambel town in #Idlib western countryside, #WhiteHelmets teams worked to rescue the injured and extinguish the fires caused by the shelling. #Syria pic.twitter.com/LBFRC13AQ5— The White Helmets (@SyriaCivilDef) 4 вересня 2018 р.
3 women were injured, after an aerial bombardment struck the small village #Pedershmu, in the western countryside of #Idlib. #WhiteHelmets quickly arrived on the scene to extinguish the resultant fires in the homes and transfer the wounded by ambulance. #Syria pic.twitter.com/cc9IvnmPc4— The White Helmets (@SyriaCivilDef) 4 вересня 2018 р.