Syrian soldiers clashed with deserters on Saturday in the restive central town of Rastan, the Syrian Observatory for Human Rights said.
"Violent clashes are under way in the city of Rastan," pitting the army and the regime's security forces against groups of deserters, the Britain-based Watchdog said in a statement received in Nicosia.
There was no immediate word on casualties.
The regime's forces used heavy weapons, and explosions were heard in the city 160 kilometres (100 miles) north of the capital Damascus, the Observatory said.
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Rastan saw similar fighting in late September when it was the focal point of a military crackdown that killed dozens of people, activists said at the time.
In a September 30 statement, deserters said they had withdrawn from Rastan because of massive reinforcements by security forces and gangs loyal to the regime of President Bashar al-Assad.
"We decided to withdraw in order to continue the struggle for freedom," they wrote.
Damascus does not recognise the scale of the protest movement that erupted in mid-March, insisting instead that it is fighting "terrorist groups" seeking to sow chaos in the country as part of a foreign-hatched conspiracy.