The head of Lebanon's powerful armed group Hezbollah said on Wednesday that Israel's killing of the deputy chief of allied Palestinian faction Hamas in Beirut was "a major, dangerous crime about which we cannot be silent".
In a televised speech, Sayyed Hassan Nasrallah offered his condolences to Hamas for what he called a "flagrant Israeli aggression" on Tuesday night that killed Saleh al-Arouri.
Tuesday's strike hit the southern Beirut suburb of Dahiyeh, in what analysts have said could also be a message from Israel to Hezbollah that even its prime stronghold there can be reached.
It was the first strike to hit Beirut following almost three months of near daily fire between the Israeli military and Hezbollah that has been confined to the border region.
Hezbollah launched rockets across the border on Oct. 8 in support of Hamas, which had carried out a deadly assault into southern Israel the previous day that prompted a fierce Israeli bombing campaign of the Gaza Strip.
Hezbollah, founded by Iran's Revolutionary Guards in 1982, is the spearhead of a Tehran-backed alliance hostile to Israel and the United States. It fought a month-long war against Israel in 2006.
Nasrallah said Hezbollah's "quick" action on Oct. 8 and the cross-border shelling since then had prevented a broader bombing campaign by Israel of Lebanon, and had showed that Israel's ability to deter such attacks "had collapsed".
He vowed that there would be "no ceilings" and "no rules" to his group's fighting if Israel chose to launch a war on Lebanon.
"Whoever thinks of war with us - in one word, he will regret it," Nasrallah said.
More than 120 Hezbollah fighters have been killed in the Israeli bombardment, along with nearly two dozen civilians. At least nine Israeli soldiers have also been killed in exchanges of fire.
Arouri was credited with re-establishing Hamas' relationship with Hezbollah after the groups backed opposing sides in the war that erupted in Syria in 2011.
Nasrallah spoke on Wednesday to commemorate four years since the killing of Quds Force commander Qassem Soleimani in a U.S. drone strike on Baghdad.
He said the dozens killed in two explosions at a ceremony in Iran to commemorate Soleimani were "targeted". Iranian officials blamed unspecified "terrorists".
It was Nasrallah's third speech since Oct. 7. In his long-awaited first speech on Nov. 3, Nasrallah warned the United States that preventing a regional conflict was reliant on stopping the Israeli attacks on Gaza.
In his second speech roughly a week later, he said the front against Israel would remain active.
On Wednesday, he said he would speak again on Jan. 5 in more detail about Hezbollah's operations in the south.
