Supported by pro-regime militias, fighters with Lebanese Shiite movement Hezbollah and intense Russian air strikes, the army entered the town of Salma, the Syrian Observatory for Human Rights said.
Rami Abdel Rahman, head of the British-based monitor, said pro-government forces were locked in fierce fighting with rebels including Islamist hardliners Ahrar al-Sham and Al-Qaeda-affiliated Al-Nusra Front.
Over the past 48 hours, more than 120 air strikes by Russian warplanes targeted Salma and its outskirts, the Observatory reported.
According to Syria's state news agency SANA, army units and the pro-government National Defence Forces militia seized seven hilltops encircling Salma, then advanced into the town's southeastern suburbs.
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If the government recaptures Salma, which has been held by opposition forces since July 2012, army units would be able to push further into rebel-held territory in Jabal Akrad, Abdel Rahman said.
Opposition forces in Latakia province -- including Al-Nusra -- are largely based in the northern and northeastern areas of Jabal Akrad and Jabal Turkman.
Regime forces have fought fiercely in recent months to retake those areas.
Since September 30, they have been backed by an intense air campaign by Russia, a staunch ally of Assad.
More than 260,000 people have died since Syria's conflict erupted in March 2011.