Syrian government forces have solidified their grasp on the last major town held by Sunni Muslim rebels near the Lebanese border.
Government troops, backed by Lebanese Shi'ite Hezbollah fighters and local paramilitaries, bombarded areas around Yabroud on Tuesday.
A successful push into the town would help President Bashar al-Assad's military seal a link between Mediterranean coastal bastions of his minority Alawite sect and the capital, Damascus.
Syrian forces have been on the offensive in the mountainous Qalamoun region where Yabroud is located since December, attempting to sever rebel supply routes from Lebanon.
Thousands of people fled the town, about 60 kilometers north of Damascus, after it was bombed and shelled last month ahead of the assault.
Hezbollah leaders in Lebanon would like to have the region across the border in Syria cleared of Sunni Muslim rebels trying to topple Assad's government.
The Shi'ite militant group claims several cars used in recent bombings in Beirut were rigged in Yabroud and smuggled into Lebanon for attacks on Hezbollah strongholds.
The Britain-based Syrian Observatory for Human Rights said rebels fighting in Yabroud mostly belong to hard-line Islamist groups, including the al-Qaida linked Nusra Front and the breakaway Islamic State of Iraq and the Levant (ISIL).
Also Tuesday, the Dutch diplomat leading the international mission to dismantle Syria's chemical weapons program said nearly one-third of the country's chemical weapons stockpile has been removed or destroyed.
Sigrid Kaag, head of the joint U.N.-Organization for the Prohibition of Chemical Weapons Mission, said the pace of removing chemical agents is accelerating and an end-of-June deadline for the program's total destruction is achievable.
The OPCW cited the Syrian government as saying it had now handed over six consignments of declared toxic agents as part of a Russian-U.S. deal struck last year.
The group said it has confirmed two more shipments are headed for the northern Syrian port of Latakia where they will be transferred to a U.S. ship fitted with special equipment that will destroy hundreds of tons of toxins.
The OPCW said Syria has submitted a revised plan to remove all chemicals from its territory by the end of April.
Damascus missed deadlines in December and February to hand over chemicals, and diplomats are concerned it will also miss a final, politically significant deadline of mid-2014 to completely relinquish its chemical stockpile.