The Britain-based Syrian Observatory for Human Rights said around 60 people were also wounded in the air strikes that struck several areas in the northwestern province, which is Syria's last major rebel bastion.

"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," Observatory head Rami Abdel Rahman told AFP.

"These are the first Russian air strikes against the province" since an accord struck by Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan in September last year.

The truce deal was meant to stave off a planned offensive by the regime and its Russian backers that aid groups feared could spark the eight-year-old Syrian conflict's worst humanitarian crisis to date.

Under the deal, opposition backer Turkey would exert its influence over anti-regime groups in the Idlib region to have them pull back fighters and heavy weapons from a demilitarised zone.

The government assault on the last major bastion of rebel forces has been held off but the deal's provisions have not been implemented.

According to Abdel Rahman, a prison in a suburb of Idlib city was hit on Tuesday in the Russian strikes, sparking a jail break by dozens of inmates.

The Idlib region is mainly controlled by the jihadists of Hayat Tahrir al-Sham (HTS), a Syrian group led by former Al-Qaeda fighters, after they pushed back smaller, Turkey-backed rebel outfits in January.