Funerals like this one in Beirut have been taking place for Hezbollah fighters said to have been killed in Qusair
The leader of the Lebanese Shia militant Hezbollah movement, Hassan Nasrallah, has promised his supporters they will prevail in Syria, where they are backing President Bashar al-Assad.
"This battle is ours... and I promise you victory," he said.
Syrian rebels in the besieged town of Qusair say they are under heavy bombardment from Hezbollah combatants.
The town is close to the Lebanese border, a conduit for both the government and rebels to get weapons.
In a televised speech from an undisclosed location, Mr Hasrallah said if Sunni Islamists took over in Syria, they would pose a threat to the entire Lebanese population - Shia and Sunni Muslims, as well as Christians.
He said his movement could never be aligned with Syrian rebels who, in his view, were supported by the United States and Israel. Offensive intensified
Dozens of Hezbollah militants are said to have been killed fighting alongside Syrian troops in Qusair since 19 May, when government forces launched an offensive to recapture the rebel-held town.
Last week, US Secretary of State John Kerry said thousands of Hezbollah fighters were contributing significantly to the violence in Syria.
He added that Iran was actively supporting Hezbollah's involvement.
The week-long fighting in Qusair intensified early on Saturday, when activists reported heavy bombardments, including two ground-to-ground missiles and an air strike as well as artillery and rocket fire.
They say Hezbollah fighters, who are widely reported to have played a major role in the attack on Sunday, were trying to advance again.
"I've never seen a day like this since the battle started," activist Malek Ammar told Reuters news agency from the town. "It's like they're trying to destroy the city house by house."
Syrian state media said the army had launched a three-pronged offensive in the north, centre and south of Qusair, and was making big advances, killing many rebel fighters.
However, official media made no mention of the part played by Hezbollah.
The Lebanese group is also known to have lost a number of fighters in Qusair, prompting Lebanese President Michel Suleiman to warn the Shia militia against getting "bogged down in the sands of discord".
The Syrian Observatory for Human Rights, a UK-based activist group that monitors the conflict, said at least 22 people including 18 rebels had been killed in the latest fighting in Qusair. Dozens had been wounded, it added.