Heavy bombardment killed nearly 30 civilians across northern Syria on Friday, a monitor said, in some of the fiercest shelling of opposition-held areas there in months.
The air strikes and barrel bombs targeted the key opposition-held province of Idlib in Syria's northwest and a rebel town in the adjacent province of Aleppo.
Idlib is the largest chunk of territory still in opposition fighters' hands, and regime leader Bashar al-Assad has warned it would be his next target.
The province's southwest was shelled heavily on Thursday and the bombing the next day "moved further east," leaving 11 civilians dead, according to the Syrian Observatory for Human Rights.
"Air strikes by Russian warplanes and barrel bombs from Syrian helicopters hit southern parts of Idlib province today in very heavy shelling," Observatory head Rami Abdel Rahman said Friday.
"It's the most intense bombing since Idlib was declared a de-escalation zone last year," he told AFP.
The shelling left eight civilians dead in the main town of Khan Sheikhun, and another three including a child died in nearby Al-Tah.
The White Helmets, a rescue force operating in opposition-held areas of Syria, said its volunteers were responding to a bombing blitz on both Khan Sheikhun and Al-Tah.
A White Helmets rescuer in Khan Sheikhun told AFP that residential districts had been hit.
Hours later, air strikes pummelled the main rebel town of Orum al-Kubra in the neighboring province of Aleppo, the Observatory said, without being able to immediately identify if they were Syrian or Russian.
"The death toll is now at 18 civilians including three children, and dozens of people injured," said Abdel Rahman.
Rebels have lost swathes of the territory they once controlled in Syria to regime forces over the last few months, including three areas that had been designated as "de-escalation zones" last year.
Assad's troops now appear to have set their sights on the last such area, Idlib.
The Britain-based Observatory said regime reinforcements, including troops and equipment, had been amassing around the southwestern part of Idlib for several days.
But a full-fledged assault would be devastating for the estimated 2.5 million people living in Idlib, many of them opposition fighters and civilians bussed out of other areas that came back under regime control.
The United Nations appealed Thursday for talks to avert "a civilian bloodbath" in the province, which borders Turkey.
"The war cannot be allowed to go to Idlib," said Jan Egeland, head of the UN's humanitarian taskforce for Syria.