"The enemy, and those who are behind the enemy, must await our inevitable response," he said in a speech broadcast at the funeral of Hezbollah military commander Fuad Shukr.
"You do not know what red lines you crossed," he said, addressing Israel after separate strikes in Beirut and Tehran killed Shukr and Hamas political leader Ismail Haniyeh, describing the latter's killing as a "dangerous assassination".
Prime Minister Benjamin Netanyahu said on Thursday that Israel was prepared for any "aggression" following threats of retaliation for the killings of Haniyeh and Shukr.
Sources and analysts said Iran and armed groups it backs were preparing coordinated action meant to deter Israel but avert all-out war, after losing two major figures in less than 24 hours.
Nasrallah warned the group will deliver "a real response, not a symbolic" one.
Israel has not commented on Haniyeh's killing but it announced that it had "eliminated" Shukr, describing him as Hezbollah's "most senior military commander" and Nasrallah's "right-hand man".
It also blamed him for carrying out a weekend rocket attack on the Israeli-annexed Golan Heights that killed 12 children, which Hezbollah denies.
Shukr, who used the nom de guerre Hajj Mohsen, led operations in south Lebanon, where the group says it has opened a "support front", exchanging near-daily fire with Israel since war erupted in Gaza in October.
"We, on all the support fronts, have entered a new phase," Nasrallah said, referring to Hezbollah and other Iran-backed groups that have targeted Israel in support of Hamas after the Palestinian group launched an October 7 attack on Israel, triggering the war.
"The battle is open on all fronts," he said, adding: "Our escalation will depend on the enemy's behaviour and reaction".
Nasrallah said that Hezbollah, which has not claimed any new attacks since Shukr's killing, would resume operations along the border on Friday morning, but that those would not be part of its response to the killings of Shukr and Haniyeh.
"What happened in the suburb was an aggression, not just an assassination," Nasrallah said, insisting Israel targeted "a civilian building, not a military base" and "killed civilians".
The strike on the Beirut suburb, an overcrowded residential area that is also a Hezbollah bastion, also killed three women and two children, the Lebanese health ministry said.
