Lebanon's Hezbollah leader Hassan Nasrallah warned on Friday that his Iranian-backed Shiite militia would make lives of Israelis "a living hell" if it is attacked.
"There are targets in occupied Palestine (Israel) which could be targeted by a small number of missiles," Nasrallah said.
"If we are forced to use them to protect our people and our country, we will not hesitate to do so... and that will turn the lives of hundreds of thousands of Zionists into a living hell," he said in a speech marking Al-Quds (Jerusalem) Day.
Nasrallah warned of "tens of thousands of deaths, and not just 300 to 500 dead," adding that Hezbollah, which is also backed by Syria, had fixed its targets.
In case of any Israeli attack on Iran over its controversial nuclear programme, "the response will be enormous," he warned. Any such action would present the Islamic republic with "the opportunity is has been dreaming of" since it was founded in 1979.
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Last month, an Israeli general warned that Hezbollah would pay dearly if it fired rockets at the Jewish state.
"If we get to another war, Israel will hit Hezbollah decisively, quickly, as fast as we can in order to stop the fire from Lebanon to Israel," commander of the country's northern division, Brigadier General Herzi Halevi, said at a briefing.
He also said that towns in southern Lebanon used as launching bases for Hezbollah rocket attacks would be "destroyed."
Israel and Lebanon are officially in a state of war, and the Jewish state fought a devastating war with Hezbollah in the summer of 2006.
Last month, Israel accused Iran and Hezbollah of planning attacks in "over 20 countries" in remarks just days after a deadly attack in Bulgaria that killed five Israeli tourists and their local bus driver.
Iran denied the charges but Hezbollah refused to comment.
Hezbollah follows the example of its backer in marking Quds Day, a show of support for the Palestinians over the disputed holy city of Jerusalem.