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.
The fighting, which has also increased around the northern city of Aleppo and seen heavy government air strikes in Homs province, undermines a weeks-old ceasefire deal and threatens to derail UN-led peace talks.
The fresh insurgent attacks came after rebel groups announced a new battle against government forces which they said was a response to violations of the truce.
Warring sides have traded blame over violations of the 27 February cessation of hostilities deal brokered by Washington and Moscow. It excludes Islamic State and al-Qaeda's Nusra Front.
In Latakia's northern countryside, a number of insurgent groups launched a fierce attack on government forces and their allies in early morning and had by midday made advances, the Observatory said.
In Hama, insurgents had almost completely taken over the town of Khirbat al-Naqus in the strategic Ghab plain, the Observatory reported. The groups attacking there included factions that did not commit to the truce.
A Syrian military source confirmed the insurgent attacks in the west. The source said the army was fighting back and had thwarted a car bomb attack in the Ghab plain.
"Today they attacked in the northern Latakia countryside in several areas, in violation of the cessation of hostilities agreement, and also in the northwestern Hama countryside," the source said. Violent clashes continued, the source said.
The Observatory reported that in northern Homs province heavy government air strikes killed four people, and said the death toll was expected to rise with more people wounded.