Syrian regime forces made gains Sunday in the key town of Nabuk, one of the last rebel-held areas in the Qalamoun region bordering Lebanon, a watchdog said.
"There is fierce fighting in Nabuk between government forces, backed by Lebanese Hezbollah fighters, and Al-Nusra Front and the Islamic State of Iraq and the Levant," the Syrian Observatory for Human Rights said.
The monitor, which relies on activists and medics on the ground for its information, said President Bashar al-Assad's troops have taken "new sectors of the town".
Regime forces have surrounded and pounded Nabuk for the past two weeks.
"The Syrian army is continuing to rake orchards in Nabuk, and has discovered a terrorist lair containing medical equipment and drugs," state television said.
"Terrorists" is the term used by Damascus to refer to the armed opposition battling to bring down Assad's regime for nearly three years.
Taking the Nabuk area would cement regime control of territory linking Damascus province with Homs province in central Syria.
In northern Syria, meanwhile, the death toll from regime air raids on the jihadist-held town of Raqa has risen to 18, the Observatory said, updating an earlier toll of 14.
Five women and six people under the age of 18 were among the dead, the Britain-based group said.
Since March 2011, when initially peaceful protests broke out against Assad, at least 126,000 people have been killed in the violence ravaging Syria, it says.