Sheikh Naim Qassem told the BBC that "very serious and very dangerous developments could occur in the region, and no-one would be able to stop the repercussions".
Hezbollah's deputy leader was speaking in an interview in Beirut, as the Hamas-run health ministry in Gaza said more than 10,000 people had been killed there.
Israel's assault follows the Hamas attacks on 7 October which killed 1,400 people - 1,000 of them civilians.
"The danger is real," he said, "because Israel is increasing its aggression against civilians and killing more women and children. Is it possible for this to continue and increase, without bringing real danger to the region? I think not."
He insisted any escalation would be linked to Israel's actions. "Every possibility has a response," he said.
Hezbollah, "the Party of God" has plenty of possibilities.
The Shia Islamist group - classed as a terrorist organisation by the UK, US and the Arab League - is the largest political and military force in Lebanon.
So far its response to the war in Gaza has involved amplifying its warnings, but carefully calibrating its actions.
When an Israeli strike killed a woman and three children in southern Lebanon on Sunday, Hezbollah used Grad rockets for the first time in the conflict, killing an Israeli civilian.
Hezbollah's leader, Hassan Nasrallah, has threatened that every civilian death in Lebanon will reap another across the border. But notably, he has not threatened Israel with all-out war
While insisting that "all options are on the table" the militant group has confined itself to cross-border attacks, hitting mainly military targets. More than 60 of its fighters have been killed, but it has plenty more battle-hardened supporters to replace them. One fighter buried in Beirut this week was the fifth member of his family to die for Hezbollah, going back generations.
Throughout our interview the organisation's deputy leader tried to portray Hezbollah as a defensive organisation - though it is committed to Israel's destruction and sparked a war with Israel in 2006 by abducting two of its soldiers in a cross-border raid.
Sheikh Qassem claimed Israel "initiated the aggression against Gaza in a hideous way".
When the BBC pointed out that it was Hamas that had attacked Israel on 7 October, he defended the attacks as an inevitable response to Israeli occupation of Palestinian lands.
He repeated the unfounded claim that Israeli forces, not Hamas, killed many Israeli civilians. But what of the helmet cameras - worn by the Hamas militants themselves - showing them on a killing spree?
He parried the question. "Why don't we look at what Israel has done inside Gaza," he said. "They kill civilians and demolish homes."
He called the Hamas attacks "a great result for the Palestinian resistance" and denied they had backfired. What about the 10,000 Gazans who have been killed since then? "The massacres committed by Israel are mobilising the Palestinians more and more to cling to their land," he replied.
He conceded that Iran "supports and finances" Hezbollah but claimed it did not give the orders. But experts say it is Tehran that calls the shots and will decide whether or not to engage in all-out war.
And if Israeli forces have to wage war on a second front with Hezbollah, they will be facing an enemy with more arms than most countries. The militant group puts Hamas in the shade, with an estimated 150,000 rockets and missiles.
It has up to 60,000 fighters, including special forces, regular fighters, and reserves, according to Nicholas Blanford, a Beirut-based defence and security consultant, who has studied Hezbollah for decades.
Back in 2006 the group fought Israel to a standstill, but Lebanon had a lot more dead. More than 1,000 of its people were killed, most of them civilians, and whole neighbourhoods were flattened in Hezbollah strongholds. Israel lost 121 soldiers and 44 civilians.
Lebanon has careened from crisis to crisis since then - with the devastating explosion in Beirut port in 2020, the collapse of the economy, and the disintegration of the political system. Small wonder few here have an appetite for war.
Many worry that Hezbollah's cross-border attacks could drag this country into a war it cannot afford. Sheikh Qassem is unapologetic. "It's the right of any Lebanese to be afraid of war," he said. "That's normal. Nobody likes war. Tell the Israeli entity to stop the aggression, so the battles do not expand."
There could be many shades of escalation ahead - short of all-out war between Hezbollah and Israel. But if it comes it will bring devastation all round, says Blanford.
"It's going to make what's happening in Gaza look like a walk in the park," he told the BBC.
"Israel will be in lockdown for the duration of the conflict. Most of its population will have to remain in bomb shelters," he said.
"There would be no civil aviation or maritime traffic. Hezbollah's larger guided missiles could hit military targets across the country."
As for Lebanon, he said Israel would reduce it to "a car park".
For now, Hezbollah, Israel, and Iran are all holding back, old enemies assessing new realities.
That doesn't mean all-out war won't happen - by miscalculation if not by design.
This is a dangerous new chapter in a blood-soaked region. After 7 October, the only certainties appear to be more anguish, death, and destruction.
The second in command of Hezbollah - the powerful Iranian backed militia in Lebanon - has said Israel's killing of civilians in Gaza risks wider war in the Middle East.