Nasrallah's comments in a television broadcast Tuesday night indicates a widening of a regional sectarian conflict between Islam's mainstream Sunni sect and the smaller breakaway Shiite branch, with Syria the main crucible.
This comes at a time when the possibility of Western intervention to aid anti-Assad forces has increased.
Nasrallah confirmed that Hezbollah fighters, veterans of years of battling Israel, are fighting rebel forces in Syria.
That was the first high-level acknowledgement by the Iranian-backed movement, the most powerful military force in Lebanon, that it's engaged in combat in Syria, although it's been common knowledge in Lebanon for some months.
Hezbollah casualties aren't known with any degree of certitude but scores of fighters killed in combat have been quietly buried in their hometowns and villages in Hezbollah strongholds in recent months, without any of the high-profile ceremony accorded those who fell fighting Israel.
The fact that Nasrallah has confirmed Hezbollah's combat role in Syria, after seeking to veil them for so long, indicates that the movement's ruling council, heavily influenced by Tehran, is preparing for expanded military operations against Assad's enemies.
It's unclear where these will be but so far Hezbollah's main combat activity has been in western Syria holding a land supply corridor from the movement's strongholds in northeastern Lebanon to regime-held territory around the city of Homs, and in part of Damascus where rebel forces are pressing the regime hard.
One sector that is of great importance to Hezbollah's Shiite fighters is a southern suburb of the Syrian capital where the tomb of Sayida Zeinab, granddaughter of the Prophet Muhammad and one of Shiite Islam's holiest shrines.
Nasrallah said rebel forces have captured several villages near the shrine and hold positions only a few hundred yards from it. Hezbollah forces have been defending the shrine for weeks.
The Hezbollah chief heightened concerns of the increasingly sectarian nature of the war between predominantly Sunni rebels and Assad regime commanded by Syria's Alawite minority, an esoteric Muslim sect with strong links to Shiism.
If the shrine is destroyed, things will get out of control," Nasrallah warned. He vowed "dangerous retribution" if any harm befalls the shrine.
Nasrallah's address was broadcast by Hezbollah's al-Manar TV channel from one of several hideouts he's been using since Hezbollah's 2006 war with Israel because of fears of assassination by the Jewish state's intelligence services.
It was aired as Nasrallah was reportedly arriving in Tehran where he had flown supposedly to attend an Islamic conference. But he'll no doubt be conferring with Iran's military and security chiefs, who've been aiding Assad's regime.
Nasrallah hinted at a possible widening of the Syrian conflict, in which some 75,000 people have been killed since it began March 15, 2011.
He suggested Russia and Iran, Assad's two key allies outside Hezbollah, would intervene with military force to prevent the downfall of the Damascus regime -- a hint things are going badly for Assad.
"Syria has true friends in the region and the world that will not let Syria to fall in the hands of America or Israel or takfiri (jihadist) groups," Nasrallah declared.
"Nasrallah just made sure Syria will get a lot worse," observed Emile Hokayem of the International Institute for Strategic Studies in London.
Stepping up Hezbollah's presence in Syria will almost certainly intensify the destabilization of Lebanon because of the war. The country's wracked by political crisis, unable to form a new government because of sectarian rivalries.
There's a palpable anger swelling among Lebanon's Sunnis against the Shiite movement which, as a Syrian ally since its creation in 1982, plunged the country into a destructive war in 2006 and now seems to be doing the same again.
Lebanese Sunnis have been aiding the Syrian rebels, which has heightened tensions.
But the traditional Sunni leaders haven't been able to contain, let alone resist, Hezbollah's growing power.
This is driving many people, who see Hezbollah as a tool of Iran, into the arms of more militant Sunni Islamists. That raises the prospect of a showdown between Hezbollah and Sunni hard-liners linked to al-Qaida.