Air strikes carried out by the Syrian regime and Russia targeted multiple towns in Idlib and Hama provinces.
Syrian regime and Russian fire on Monday killed over 25 civilians, including several children in different parts of northwest Syria, a Britain-based war monitor said.
The Syrian Observatory for Human Rights said over half of those slain were killed in regime air raids on the town of Jabala in Idlib province.
The attacks also targeted other towns in both Idlib and Hama provinces.
Turkey and Russia agreed last September to turn Idlib into a de-escalation zone in which acts of aggression would be expressly prohibited.
The Syrian regime, however, has consistently broken the terms of the ceasefire, launching frequent attacks inside the de-escalation zone.
Syria has only just begun to emerge from a devastating conflict that began in early 2011 when Bashar al Assad's regime cracked down on demonstrators with unexpected severity.