The dead include at least eight children, White Helmets Syria Civil Defence says.
Syrian regime shelling and Russian air strikes killed at least 15 civilians on Wednesday in Idlib province, in the first such raids since a September truce deal, a monitor said.
At least eight of the casualties were children, White Helmet Syria Civil Defence added.
The Britain-based Syrian Observatory for Human Rights (SOHR) said around 60 people were also wounded in the air strikes that struck several areas in the northwestern province.
A harrowing search and rescue operation carried out by the #WhiteHelmets in #Idlib City rescued a woman alive. The city was attacked with four air raids resulting in 15 people killed, including 8 children and two women, and 49 people were injured. #Syria pic.twitter.com/dV0jgKcHY5— The White Helmets (@SyriaCivilDef) March 14, 2019
"Over the past 24 hours Russian aircraft have carried out dozens of raids targeting several regions in Idlib province, including the city of Idlib and the town of Saraqeb," SOHR head Rami Abdel Rahman said.
"These are the first Russian air strikes against the province" since an accord struck by Turkish President Recep Tayyip Erdogan and his Russian counterpart Vladimir Putin in September last year.
TRT World's Yasin Eken reports.
In September 2018, Turkey and Russia agreed to turn Idlib into a demilitarised zone after a meeting between Erdogan and Putin, in Sochi, Russia.
Ankara and Moscow signed a memorandum of understanding calling for the stabilisation of the situation in Idlib's de-escalation zone, in which acts of aggression are prohibited.
Idlib has been in the hands of rebels for years, even as Assad's regime has taken back other rebel enclaves.
The province is now home to some three million people, many of them displaced from other areas held by rebels or the opposition.