Syrian troops advanced on a major rebel-held town near the Lebanese border, as the country's bloody conflict marked its third anniversary Saturday.
State media and activists said soldiers supported by warplanes, artillery and tanks entered the town of Yabroud.
The Syrian Observatory for Human Rights reported fierce clashes between government forces and rebel factions.
Yabroud is the rebels' last stronghold in the Qalamoun region, where government forces have been waging an offensive to try to sever rebel supply lines across the Lebanese border.
More than 140,000 people have been killed and another 2.5 million have fled the country since Syria's civil war began in March 2011. The conflict started with mass street protests against Syrian President Bashar al-Assad and turned into an insurgency after a violent crackdown on demonstrators.
In other news, the Syrian Observatory for Human Rights said a top commander of the al-Nusra Front, an al-Qaida-linked group, was killed on Friday in Yabroud during clashes between the army and Hezbollah fighters.
The observatory said Abu Azzam al-Kuwaiti was a leader of al-Nusra in the Qalamoun region, saying he helped negotiate a prisoner swap that saw rebels recently release more than a dozen Christian nuns.
Hezbollah, the Lebanese militant group, has been fighting in the war alongside forces loyal to Mr. Assad.