Syrian forces and their Russian backers unleashed heavy air strikes on rebel-held Idlib on Friday, a monitor said, expanding their shelling of the northwestern province.
Idlib is the largest chunk of territory still in rebel hands, and President 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", said 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 told AFP on Friday.
Shelling killed two civilians in the main town of Khan Sheikhun, and another two died including a child in nearby Al-Tah, he said, adding that dozens of people were wounded.
"The barrel bombs were focused on Khan Sheikhun," he added.
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.
Assad's troops appear to have set their sights on Idlib after making sweeping military gains across Syria in recent months, including around Damascus and in the south.