Rebels launched a fierce attack against Syrian government forces in Latakia province on Monday and insurgents made separate advances nearby in Hama, a monitoring group reported, as violence escalated in the west of the country.
Heavy government air strikes in Homs province meanwhile killed four people, and the death toll was expected to rise with more people seriously wounded, the British-based Syrian Observatory for Human Rights said.
Rebel groups earlier announced a new battle against government forces which they said was a response to violations of a truce deal reached several weeks ago.
It was not immediately clear if the Latakia assault was part of that new battle. In Hama, insurgents had almost completely taken over the town of Khirbat al-Naqus, the Observatory reported.
The groups attacking in the strategic Ghab plain in Hama province included factions that did not commit to the Feb. 27 cessation of hostilities agreement brokered by Washington and Moscow.
The truce also excluded Islamic State and the al-Qaeda-affiliated Nusra Front.