Hasan Nasrallah, the leader of Hezbollah, says the killing of Hamas's powerful deputy leader will "not go unpunished".
Saleh al-Arouri died in a drone attack in Beirut on Tuesday. Israel has not confirmed it was responsible.
Mr Nasrallah described Arouri's death as a "major, dangerous crime about which we cannot be silent".
He added that if Israel wages war on Lebanon "there will be no ceilings, no rules" to Hezbollah's response.
"We are not afraid of war," said Mr Nasrallah in a televised address.
"Those who think of going to war with us will regret it. War with us will come at a very high cost."
He previously said that any killings in Lebanon would be a red line.
Hezbollah - like its ally, Hamas - is considered a terrorist organisation by Israel, the UK and others. It is the largest political and military force in Lebanon and has ministers in the country's government.
The attack that killed Arouri was the first in Beirut following months of exchanges of fire between Iranian-backed Hezbollah and Israel, in response to the war in Gaza. These had largely been confined to Lebanon's southern border.
The Israeli Defense Forces (IDF) said on Wednesday it was "defending its borders" from a number of missiles launched from Lebanon.
That was after its military was reported to have increased the number of its air defence batteries near the border.
Arouri was a key figure in the Izzedine al-Qassam Brigades, Hamas's armed wing, and a close ally of Ismail Haniyeh, the Hamas leader. He had been in Lebanon acting as a connection between his group and Hezbollah.
He was known to be on Israel's hit list and the country's prime minister, Benjamin Netanyahu had previously vowed to eliminate Hamas's leaders - wherever they are.
Arouri was on a US blacklist too, with a bounty of $5m ($3.9m) on his head since 2018.
Miri Eisin, a retired colonel from the Israel Defense Forces (IDF) told the BBC that the Hamas deputy was "living in the heart of the Dahiya, in the heart of the Hezbollah area of Beirut" and that he was the "direct connection between Hamas and Hezbollah".
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