Hezbollah chief Hassan Nasrallah acknowledged Thursday his powerful group had suffered an "unprecedented" blow when thousands of operatives' communication devices exploded in attacks it blamed on Israel.
Israel has not commented on the attacks that killed 37 people and wounded nearly 3,000 across Lebanon over two days but has said it will widen the scope of its war in Gaza to include the Lebanon front.
Delivering a speech after the attacks on Tuesday and Wednesday, which plunged Lebanon into panic, Nasrallah struck a defiant tone, warning that Israel would receive "just punishment" for the attacks.
Describing the attacks as a possible "act of war", he said Israel would face "tough retribution and just punishment, where it expects it and where it does not".
"It could be a war crime or a declaration of war," he said of the attacks, which he branded a "massacre".
Nasrallah also vowed to keep up Hezbollah's fight against Israel until a ceasefire in Gaza is reached.
"The Lebanese front will not stop until the aggression on Gaza stops" despite "all this blood spilt," he said.
Nasrallah addressed Israeli officials' promises to return thousands of Israelis displaced by exchanges of fire across the border with Lebanon to their homes.
"You will not be able to return the people of the north to the north," he said, warning that "no military escalation, no killings, no assassinations and no all-out war can return residents to the border".
Hezbollah is an ally of Palestinian militant group Hamas, which on October 7 launched an unprecedented attack on Israel that sparked Gaza's deadliest ever war.
Up until now, the focus of Israel's firepower had been on Gaza.
But Israel's northern border with Lebanon has seen exchanges of fire between Israeli troops and Hezbollah militants almost every day since October.
The violence has killed hundreds of people, mostly fighters, on the Lebanese side, and dozens on the Israeli side.
Israeli warplanes broke the sound barrier over Beirut as Nasrallah spoke, Lebanon's state-run National News Agency said, with AFP correspondents in Beirut reporting loud booms.
Nasrallah announced the launch of an internal probe into the attacks, which experts and some Israeli media have said bear all the hallmarks of Israeli intelligence agency Mossad.