Overnight clashes between security forces and army defectors in northern Syria left 12 soldiers and defectors and three civilians dead early on Saturday, activists said.
Fighting has become more intense as rebels increasingly often confront security forces loyal to President Bashar al-Assad who are trying to suppress the eight-month-old protest movement against his rule.
The Syrian Observatory for Human Rights said fighting broke out around midnight in the northern city of Idlib, near the Turkish border.
Seven were killed from the army and regime security forces, including an army officer, the group said. Three civilians and five defectors were also killed.
More than 4,000 people have died since protests broke out in March against the Assad family's 41-year rule, according to the United Nations, which says the violence in Syria looks like civil war.
Syria faces mounting international and regional isolation as organisations such as the Arab League and the European Union, and the United States, demand that Damascus stop the bloodshed and talk to its opponents, and impose increasingly tough sanctions when it does not do so.
Syrian authorities say they are fighting foreign-backed terrorist groups trying to spark civil war who have killed some 1,100 soldiers and police since March.
The head of the main group of army deserters who have joined the opposition, the Free Syrian Army, told Reuters that his forces were switching their tactics from seizing equipment and hitting security checkpoints to attacking the military directly.
He said this was a necessary response to an increasing use of violence in Damascus' military crackdown on protests.
(Editing by Douglas Hamilton and Tim Pearce)
(Reporting by Erika Solomon)