The leader of Lebanon's Hezbollah has said his fighters would expand their presence in Syria, saying the group was engaged in an existential battle against the Islamic State of Iraq and the Levant (ISIL).

Hassan Nasrallah acknowledged for the first time that his Shia group was fighting across all of Syria in support of President Bashar al-Assad's government.

He called specifically on his fiercest critics in Lebanon to back his intervention across the border, warning that their support for Assad's opponents would not save them from ISIL.

"Today we are facing a kind of danger that is unprecedented in history, which targets humanity itself," Nasrallah said, speaking ahead of Monday's anniversary of the withdrawal of Israeli troops from Lebanon in 2000.

"This is not a threat to the resistance in Lebanon or to one sector of the regime in Syria or the government in Iraq or a group in Yemen," he added, addressing an audience in the southern town of Nabatiyeh in a telecast broadcast on a big screen.

"This is a danger to everyone. No one should bury their heads in the sand.

"We invite everyone in Lebanon and the region to take responsibility and confront this danger and end their silence and hesitation and neutrality."

The speech was a full-throated defence of Hezbollah's role in Syria, where it has acted as a key force multiplier for Assad's embattled government since an uprising that began in March 2011.

The intervention has raised tensions in Lebanon, where many Sunnis back the uprising against Assad and accuse Hezbollah of drawing the country into Syria's war.

But Nasrallah has always framed Hezbollah's intervention as protecting Lebanon from the threat of extremism.

And on Sunday, he said the choice in Syria is between ISIL and al-Qaeda affiliate al-Nusra Front, or the government and its allies like Hezbollah.

'Present in all places'

Nasrallah dismissed the US-led coalition fighting against ISIL, saying the Sunni armed group continued to move freely despite the coalition's air strikes.

He directed much of his speech to members of Lebanon's Future movement, which is fiercely opposed to Hezbollah and its role in Syria, warning they would be the "first victims of ISIL and al-Nusra" if they arrive in Lebanon.

He said Hezbollah was fighting throughout all of Syria, and not just in areas near the border with Lebanon.

"We are fighting alongside our Syrian brothers, alongside the army and the people and the popular resistance in Damascus and Aleppo and Deir Ezzor and Qusayr and Hasakeh and Idlib," he said.

"We are present today in many places and we will be present in all the places in Syria that this battle requires."

Former prime minister Saad Hariri, who heads Lebanon's anti-Hezbollah bloc, criticised Nasrallah's speech and his movement's intervention in Syria.

"We in the Future Movement declare publicly that the Lebanese state and its institutions are legitimate and our choice and guarantee," he said in a statement.

"Defending the land and the sovereignty and dignity [of Lebanon] is not Hezbollah's responsibility ... and our position on Daesh [ISIL] and the forces of terror does not need to be certified by anyone."

More than 220,000 people have been killed in Syria since the conflict there began with anti-government protests before spiralling into a civil war after a government crackdown.

Assad's government and allies refer to all those seeking to overthrow them as "terrorists", and have pointed to the emergence of ISIL and other Sunni armed groups as proof that they are fighting a "war on terror".

While Lebanon is officially neutral on the conflict next door, the country has been unable to escape the effects of the war.

It hosts more than 1.2 million Syrian refugees, and has seen existing sectarian tensions rise during the war, with the population divided between support for Assad and support for the uprising.