An Israeli strike on Tuesday killed Saleh al-Aruri, deputy head of the Hamas movement, in a southern Beirut stronghold of Hamas ally Hezbollah, two security officials told AFP.
Hamas later confirmed Aruri's death, which Lebanese state media said came in an Israeli drone strike that killed a total of six people.
The attack marked an escalation of the nearly three-month war in Gaza between Hamas and Israel. Aruri, one of Hamas's principal military strategists, is the first senior official of the movement killed during the war, and his death came in the first strike on the Lebanese capital since hostilities began.
There have been regular cross-border exchanges of fire over Lebanon's southern border between Israeli forces and Hezbollah.
Asked about the Beirut strike, the Israeli military said it "does not comment on foreign media reports."
A high-level security official in Lebanon told AFP that Aruri was killed along with his bodyguards.
Another security official confirmed the same information, adding that two floors of the targeted building and one car were damaged.
Hamas later confirmed the death on its official TV channel, saying Aruri was killed in a "treacherous Zionist strike".
The movement's media said the strike killed two other members of its armed wing.
A Lebanese security official told AFP that Samir Fandi, another Hamas official, was among the dead.
Lebanese state media said "a hostile Israeli drone targeted a Hamas office in Al-Musharrafiya," in Beirut's southern suburbs, where Palestinian factions were meeting, Lebanon's National News Agency (NNA) said.
An AFP photographer said two floors of the building, on a busy street, had been blown out in a blast which sent debris into cars and buildings up to about 100 metres (yards) away.
Onlookers filled the street and red and blue emergency lights flashed.
He was elected deputy to Hamas chief Ismail Haniyeh in 2017, before being officially named the group's number two.
After spending nearly two decades in Israeli prisons, Aruri was freed in 2010 on the condition he went into exile.
Senior Hamas official Izzat al-Rishq said in a statement that Aruri's killing will not "undermine the continued brave resistance" in Gaza and "proves once more the utter failure" of Israel's goals there.
Islamic Jihad, another armed group fighting alonside Hamas in Gaza, reacted similarly to the "assassination" of Aruri "and his comrades."
In a statement, the group said the killing is an attempt to "drag the entire region into war, to escape from its failure in the military field in the Gaza Strip and the political deadlock which the entire government is experiencing."
Lebanon's Prime Minister Najib Mikati condemned the killing which he said "aims to draw Lebanon" further into the Israel-Hamas war, according to a statement from his office.
