An Israeli strike on Hezbollah's stronghold in the southern suburb of Beirut on Tuesday killed top commander Fuad Shukr, along with an Iranian military adviser and five civilians.
Israel said Shukur was behind a rocket attack days earlier that hit a football pitch in the Israeli-held Golan Heights, killing 12 children.
Hezbollah denied being behind the strike.
The killing of Shukur was the most serious blow to the Iran-backed group in nearly two decades and threatened to push the tit-for-tat exchanges across Lebanon's southern border in parallel with the Gaza War into a full-blown regional conflict.
Speaking in a televised address to mark the funeral of the slain commander, Nasrallah said the conflict had entered "a new phase unlike the previous one" and that Israel had crossed red lines with its attack on the group's stronghold.
Nasrallah said unnamed countries had asked his group to retaliate in an "acceptable" way - or not at all.
But he said it would be "impossible" for the group not to respond.
"There is no discussion on this point. The only things lying between us and you are the days, the nights and the battlefield," Nasrallah added in a threat to Israel.
He said the group had ratcheted down its operations over the last two days out of respect for the victims of the strike but would "go back to work normally starting tomorrow morning," although the retaliation for Shukr's killing would come later.
"The response will come, whether spread out or simultaneously," he said.
We eliminated Fuad Shukr. Here's why: — Israel Defense Forces (@IDF) pic.twitter.com/bxQ5WqYfrlJuly 31, 2024
Just hours after Shukr's killing, the leader of Hamas Ismail Haniyeh was killed in the Iranian capital Tehran in an attack widely blamed on Israel.
Nasrallah said that anyone seeking to prevent the region from slipping into a tailspin should work on a Gaza ceasefire.
"There will be no solution here except to stop the aggression on Gaza," he said.
Israel and Iran risked plunging into war earlier this year when Israel hit Iran's embassy in the Syrian capital Damascus in April.
Iran retaliated, and Israel countered in an unprecedented exchange of strikes on each other's soil, but international efforts succeeded in containing that cycle before it spun out of control.
In Beirut's southern suburbs, the biggest Shi'ite district in the capital, hundreds of black-clad mourners packed the auditorium, many of them holding Hezbollah flags or photos of Shukr.
An escort of red-capped fighters carried Shukr's coffin, also draped in a Hezbollah flag, down the aisle to the backing of a military band.
In his speech, Nasrallah praised Shukr as a veteran commander and denied that Hezbollah carried out a deadly strike on a football pitch in the mainly Druze town of Majdal Shams in the Golan.
"We have the courage to take responsibility for where we strike, even if it's a mistake. If we made a mistake, we would admit and apologise," he said, adding, "The enemy made itself the judge, jury, and executioner without any evidence".
An unusual relative calm prevailed on Thursday on the Lebanon-Israel border.
Hezbollah claimed no rocket launches into Israel during the day.
The Lebanese state news agency said a strike hit the house of a Syrian family in a southern Lebanese town, killing at least four people and wounding several others.
Palestinian civil emergency services said on Thursday that Israeli forces hit a school in Shejaia in Gaza City, killing at least 10 people.
The Hamas movement's al-Aqsa television said as many as 15 people were killed in the strike, which came as Israeli forces have continued battling Palestinian fighters in various parts of the Gaza Strip.
The military said it had targeted fighters operating in a compound within the school that it said was used as a hideout for Hamas commanders and fighters.
"Prior to the strike, numerous steps were taken to mitigate the risk of harming civilians, including the use of precise munitions, surveillance, and additional intelligence," it said.
with AP
27°C