The leader of Lebanon's Hezbollah has warned that the fall of Syrian president Bashar Assad's regime would give rise to extremists and plunge the Middle East into a "dark period".

He vowed his Shiite militant group will not stand idly by while its chief ally in Damascus is under attack.

In a televised address, Sheikh Hassan Nasrallah said Hezbollah members are fighting in Syria against Islamic extremists who pose a danger to Lebanon, and pledged that his group will not allow Syrian militants to control areas that border Lebanon.

Nasrallah's comments marked the first time he has publicly confirmed his men were fighting in Syria, and were his first remarks since Hezbollah fighters have become deeply involved in the battle for the strategic Syrian town of Qusair near the Lebanese frontier.

Hezbollah has come under harsh criticism at home and abroad for sending fighters to Syria to fight alongside Assad's forces. In his speech, Nasrallah sought to defend the group's deepening involvement, and frame its fight next door as part of a broader battle against Israel.

He also portrayed the fight in Syria as an "existential war" for anti-Israel groups including Hezbollah.

"Syria is the back of the resistance, and the resistance cannot stand, arms folded while its back is broken," Nasrallah told thousands of supporters from a secret location though a video link.

"If Syria falls into the hand of America, Israel and takfiris, the resistance (Hezbollah) will be besieged and Israel will enter Lebanon and impose its will," Nasrallah said. Takfiri Islamists refers to an ideology that urges Sunni Muslims to kill anyone they consider an infidel.

"If Syria falls in the hands of America, Israel and the takfiris, the people of our region will go into a dark period," he said in a speech to mark the anniversary of Israel's withdrawal from southern Lebanon in May 2000. "If Syria falls, Palestine will be lost."

Syria, along with Iran, has been the main backer of Hezbollah and much of the group's arsenal consisting of tens of thousands of rockets is believed to have come through or from Syria.

More than 70,000 people have been killed and several million displaced since the uprising against Assad erupted in March 2011 and escalated into a civil war.

The Syrian government and Hezbollah deny there is an uprising in Syria, portraying the war as a foreign-backed conspiracy driven by Israel, the US and its gulf Arab allies.