The assault is the latest step in Assad and Hezbollah's campaign to assert control over the Lebanese-Syrian border region and fortify the president's hold on central Syria, from the capital Damascus to his stronghold on the coast.
Syrian state media said the army had seized the nearby village al-Jarajeer, while rebels said Assad's forces had advanced on the area but had not completely taken it.
The military push came as international peace talks in Geneva seized up in mutual recrimination, with the government resisting discussion of a post-Assad transition while the opposition called for a UN-monitored ceasefire.
There has been little let-up in fighting despite the start of the first peace negotiations three weeks ago after nearly three years of war. Assad's forces seem to have had the better of recent fighting, but outright victory seems out of reach.
As US National Intelligence Director James Clapper put it to senators on Tuesday, a "prolonged stalemate" seems likely, extending what he described as "an apocalyptic disaster" in Syria.
Mountain battleOn Wednesday, more than 13 air strikes hit the government's target area around Yabroud in the frontier mountains, the British-based Syrian Observatory for Human Rights said.
Overnight clashes between Assad's forces and the opposition on the outskirts of Yabroud continued into the morning.
A spokesman for the rebel unit Liwa al-Ghuraba, said Hezbollah fighters and Assad forces were trying to position themselves on nearby hilltops to attack Yabroud.
"They are gathering their forces with the hope of taking the border road," said spokesman Abu Anas, speaking by Skype. "Right now no one is moving in Yabroud. The rebels are blocking the offensive ... The hospital is filling up with wounded."
Lebanese media said dozens wounded in Syria had been sent to Lebanese hospitals as well.
The attack on Yabroud is part of what locals have called the "Battle for Qalamoun", the name of a mountainous region along the frontier with Lebanon used by both the rebels and Assad's allies to bring in people and supplies.
Assad's forces sent in envoys in the days leading up to the attack to try to convince leading citizens in nearby towns to accept a truce. Some villages accepted, but most towns, like Yabroud, refused, said the rebel Abu Anas.
"The battle for Qalamoun was supposed to just be a propaganda campaign," he said. "But the regime got itself in a mess: The army sent people to convince us there could be a peaceful solution if we raised the government flag and took photos. Instead, we refused."
Activists in rebel-held besieged areas elsewhere who have accepted similar conditions in the past, in exchange for allowing in food and supplies, have said the truce ended up being more of a surrender, with little aid being allowed in.
Concern about talks running into the sand prompted the international mediator in Geneva, Lakhdar Brahimi, to bring forward by a day to Thursday a meeting with Russian and US officials in an apparent attempt to get Washington, which backs the rebels, and Assad's ally Russia, to press their proteges.
Continued strains between Russia and other world powers that have so far blocked UN action against the Syrian government showed little sign of easing. Russia said it would veto a UN resolution on aid, saying its wording seemed meant to open the way for foreign military action.
Sectarian struggleThe struggle on the Syrian border risks fuelling sectarian tensions in Lebanon, where Sunni-Shi'ite divisions deepened by the conflict in Syria have already triggered instability.
Rebels fighting to end four decades of Assad family rule are led largely by Syria's Sunni Muslim majority and have strong support from Sunnis in neighbouring Lebanon. Many of the fighting units in Yabroud are Islamist, including some with links to al-Qaeda.
Assad's forces have support from minorities, particularly his Alawite sect, an offshoot of the Shi'ite Islam practised in Iran. Assad's campaign in central Syria gained a huge boost from the support of Shi'ite Hezbollah's experienced fighters, who battled Israel in a 2006 war.
The violence in Syria has set off a wave of tit-for-tat car bombings in Lebanon by both sides, as well as sporadic street clashes. On Wednesday, Lebanese security sources said the army had arrested an al Qaeda militant they called the "mastermind of car bombs" in Shi'ite areas.
Opposition activists argue the assaults around Yabroud are also an effort to systematically push out Sunnis from Syria's mixed Sunni-Shi'ite border region.
Activists said the fresh surge in fighting sent many civilians fleeing out of Yabroud, with many heading to Lebanon.
Recently arrived refugees told the Lebanese newspaper the Daily Star that Syrian army forces warned them over mosque loudspeakers to flee the area if they wanted to save their lives.
Syria's army warned residents not to use illegal border crossings to escape, Lebanese news channel Al-Manar said, implying such routes could be targeted by the military.
Previous successful military assaults have given Assad the advantage along the Lebanese border. It was once a critical foothold for the rebels, whose main strongholds are now in Syria's northern and eastern regions as well as along parts of the southern border.
Syria's three-year conflict began as peaceful protests against four decades of Assad family rule but descended into an armed conflict after a security force crackdown.
The fighting has killed well over 130,000 people and has forced more than 6 million people to flee their homes.