Supporters of Hezbollah and relatives of Hezbollah member Saleh Ahmed Sabagh mourn during his funeral in the port-city of Sidon, southern Lebanon May 22, 2013. (Ali Hashisho/Reuters)

The leader of the Lebanese Shiite militant group Hezbollah on Saturday defiantly defended sending his fighters to support President Bashar al-Assad’s army in neighboring Syria and proclaimed that they would be victorious.

Hasan Nasrallah’s televised address provided the clearest public acknowledgment to date that his men are fighting alongside Assad’s troops and will continue to do so. As he spoke, Hezbollah and government forces were escalating an assault on the strategically important Syrian town of Qusair.

A staunch ally of Iran as well as Assad, Hezbollah has deepened its involvement in Syria’s two-year-old civil war in recent weeks, leading the push to drive rebels out of Qusair, near the Syria-Lebanon border. The group has long justified its stockpile of weapons as necessary to the “resistance” against Israel, and its growing role in Syria has stirred controversy in Lebanon.

“We will bear all responsibilities,” Nasrallah said of Hezbollah’s activities in Syria. Addressing his fighters, he added, “I have always promised you victory, and I promise victory again.”

Nasrallah framed the conflict in Syria as a struggle against an extremist opposition that is backed by Israel and the United States. He characterized Assad’s regime, which has long been a conduit for Iranian weapons transfers to Hezbollah, as the movement’s “backbone.”

“The resistance cannot stand, arms folded, while its back is broken,” he said in the speech, timed to mark the 13th anniversary of the withdrawal of Israeli troops from southern Lebanon.

Thousands of flag-waving supporters gathered to watch the Shiite cleric’s broadcast at a rally in the village of Mashghara in the Bekaa Valley, Hezbollah’s heartland. Celebratory gunfire erupted in areas of the capital, Beirut, as he spoke.

Nasrallah said it would be better for Lebanon’s stability if opposing factions crossed the border and fought in Syria. Fighting between pro- and anti-Syrian government forces in Lebanon’s northern city of Tripoli has claimed 25 lives in the past week.

He also said that if Hezbollah did not act preemptively, Lebanon would be put at further risk.

“If Syria falls into the hands of America, Israel and the takfiris, the people of our region will enter a dark period,” he said, casting the opposition as followers of an extreme interpretation of Islam that condones aggression. “If we do not go there to fight them . . . they will come here.”

Former Lebanese prime minister Saad Hariri, leader of the Sunni Future Movement, on Saturday described Hezbollah’s decision to fight in Syria as “political and military suicide.”

The fighting in Qusair has proved costly for Hezbollah, with dozens of bodies transported back to Lebanon for burial over the past week.

Ringed by Hezbollah militants and army forces, Qusair has been the focus of an intense battle for six days. Gaining control of the town is pivotal to government efforts to establish a grip on central Syria.

Activists in Qusair said the bombardment of the city Saturday morning was the heaviest yet, with as many as 30 shells falling every minute. They linked the intense push on Saturday to Nasrallah’s speech, the first since the offensive began.

“We were expecting a more decisive attempt to advance before Nasrallah’s speech, and it has happened,” said Sami al-Rifaie, an activist based in Qusair. “He wants to appear in front of his people victorious.”

Rifaie said shelling had started at around 6:30 a.m. and continued until noon. Two surface-to-surface missiles also fell on the town, he said.

“On my way home, I saw one had hit very close to my house,” he said. “I’m lost for words. I haven’t seen anything like this before. It’s a residential area, and there was destruction for a radius of 100 to 200 meters.”

Rifaie said Hezbollah militants had attempted to take control of the al-Ghaida checkpoint on the southern side of Qusair on Saturday but had been pushed back. At least 22 people, including 18 opposition fighters, were killed in the town Saturday, according to the Britain-based Syrian Observatory for Human Rights.

Suzan Haidamous in Mashghara, Lebanon, contributed to this report.