"Many cities in the centre [of Israel] will be targeted beyond Haifa if the savage aggression continues on our country, people and villages," the leader of the Shia armed group said in a televised speech on Saturday.
The threat came after Hezbollah's rockets reached the Israeli city of Afula, 50km south of the border earlier in the day - the deepest strike into Israel since the conflict began 18 days ago.
"The bombardment of Afula and its military base is the beginning," Nasrallah said an address broadcast on Hezbollah's satellite channel Al-Manar.
Commenting on the US secretary of state's arrival in the region to broker an end to the showdown, Nasrallah said,
"Rice returns to the region to impose conditions that serve Israel."
His comments came as Israel pressed ahead with its massive air and ground assault on Lebanon aimed at securing the release of two troops captured by Hezbollah in a July 12 cross-border raid.
The UN relief chief, Jan Egeland, had earlier appealed for a truce to allow casualties to be removed and food and medicine to be dispatched to war zones, saying one third of the casualties in the conflict were children.
"Rice returns to the region to impose conditions that serve Israel"
Hassan Nasrallah, Hezbollah's leader
"There is something fundamentally wrong with a war where there are more dead children than armed men. That has to stop," he said, quoting Lebanese figures that put the death toll at 600.
But the Israeli government spokesman, Avi Pazner, said, "There is no need for a 72-hour temporary ceasefire because Israel has opened a humanitarian corridor to and from Lebanon."
Aid groups have complained they have little access to south Lebanon which has borne the brunt of the Israeli onslaught.