Hezbollah leader warns Israel against waging war on Lebanon
"If the enemy thinks of waging a war on Lebanon, we will fight without restraint, without rules, without limits and without restrictions," Nasrallah said in a televised speech.
"We are not afraid of war," he said, adding that "for now, we are fighting on the frontline following meticulous calculations".
Hamas, and security officials in Lebanon, accused Israel of killing Saleh al-Aruri and six others on Tuesday.
Israeli army spokesman Daniel Hagari did not directly comment on Aruri's killing but said the military was "highly prepared for any scenario" in its aftermath.
Hezbollah and its arch-foe Israeli have exchanged near-daily fire over their border since the Israel-Hamas war broke out on October 7, but the Aruri killing led to fears of wider conflict.
He is the most high-profile Hamas figure to be killed during the war, and his death came in the first strike on the Lebanese capital since hostilities began.
The strike that killed Aruri marked "the first time they target the southern suburbs in this way since 2006," Nasrallah said, in reference to that year's Israel-Hezbollah war which saw Beirut's southern suburbs bombed.
Israel had sent "messages" to indicate it "did not intend to target Lebanon or Hezbollah," but was rather "settling scores" with Hamas leaders, the Hezbollah leader added.
Nasrallah described the attack as a "major and dangerous crime" which "will not go unanswered and unpunished" -- repeating a threat made by Hezbollah on Tuesday.
The group announced several strikes on Israeli troops and positions Wednesday, within the usual scope of the border area.
Israel's military said it responded with artillery after munitions were fired from Lebanon. Warplanes also hit Hezbollah targets and a tank struck a "terrorist squad" in Lebanon, the military said.
"Israel has been weakened" by Hamas's October 7 onslaught, Nasrallah said.
He spoke in a pre-planned speech commemorating Iran's Islamic Revolutionary Guard Corps General Qasem Soleimani four years after his death in a US strike in Iraq.
Nasrallah is set to deliver another televised speech on Friday.
Earlier Wednesday, a high-level Lebanese security official told AFP that Israel fired guided missiles from a warplane to kill Aruri. Lebanese state media had on Tuesday reported he died in a drone strike.
Hamas said Aruri would be buried on Thursday in Beirut's Shatila Palestinian refugee camp.
Since hostilities began, 171 people have been killed on the Lebanese side, most of them Hezbollah members but also more than 20 civilians including three journalists, according to an AFP tally.
On the Israeli side, at least four civilians and nine soldiers have been killed, according to figures from the military.