Assad's fall would be a dire scenario for Hezbollah. Any new regime led by Syria's majority Sunni Muslims would likely be far less friendly - or even outright hostile - to Shiite Muslim Hezbollah. Iran remains the group's most important patron, but Syria is a crucial supply route. Without it, Hezbollah will struggle to get money and weapons as easily.
The Syrian uprising has left Assad deeply isolated - making his remaining allies such as Iran and Russia all the more important. At last week's gathering of world leaders at the United Nations, dozens of nations excoriated the Assad regime for its role in a conflict that activists estimate has killed at least 30,000 Syrians.
It was not immediately clear how the Hezbollah militants were killed or whether they had been fighting alongside the Syrian army. But Hezbollah's newspaper al-Intiqad said Hezbollah commander Ali Hussein Nassif, who is also known as Abu Abbas, was killed "while performing his jihadi duties."
A Lebanese security official said Nassif was killed in Syria and his body was returned to Lebanon through the Masnaa border crossing on Sunday. Speaking on condition of anonymity, the official said the bodies of several other Hezbollah fighters had been brought back to Lebanon in recent days.
The Syrian opposition has long accused the group of helping the Syrian leadership crack down on the uprising - a claim the group has repeatedly denied. Hezbollah has to tread a careful path with its support for the regime, mindful that many of its supporters in Lebanon dread getting sucked into the conflict.
Samer al-Homsi, an activist in Syria's central Homs province, which borders Lebanon, said Nassif was killed Saturday when a roadside bomb went off as the car he was in passed just outside the town of Qusair. He said Nassif and several other people were killed in the blast.
"His job was to coordinate with Syrian security agencies," Homsi said via Skype.
Although Hezbollah's ties to Syria have stayed strong during the uprising, the government's long-standing relations with the Palestinian militant group Hamas have frayed.
Syria's state-run media unleashed a scathing attack on the leader of Hamas, accusing him of turning his back on Assad and describing him as ungrateful and traitorous.
In an editorial aired Monday, Syrian TV said Khaled Mashaal, who pulled Hamas' headquarters out of Damascus this year, had abandoned the resistance movement against Israel and the United States.
The regime's verbal attack appeared to be prompted by Mashaal's decision to take part in a major conference Sunday of Turkish Prime Minister Recep Tayyip Erdogan's ruling party. Erdogan has been one of Assad's sharpest critics.