Lebanese Shiite militant group Hezbollah will transform the lives of Israelis to "hell" if Israel attacks Lebanon, its leader said on Friday, adding that the group would not hesitate to hit targets that would leave tens of thousands of Israelis dead.

Speaking in a televised speech marking Jerusalem Day — an annual rally in support of the Palestinians — Sheik Hassan Nasrallah said only a few rockets fired by the group's militia could cause massive casualties, given its well-planned target list.

"Rockets are ready and directed at these targets," he said. "We will not hesitate to use them, if we have to, at any point in time in the course of aggression against our country to protect our people," he added.

"Hezbollah cannot destroy Israel but we can transform the lives of millions of Zionists in occupied Palestine into a real hell," the black-turbaned Nasrallah said. "We can change the face of Israel."

The threat came as Israel is undergoing an increasingly public debate on whether to attack Iranian nuclear facilities, possibly triggering retaliation from Tehran's allies such as Hezbollah. Israel considers Iran to be its most dangerous enemy and has vowed to prevent it from acquiring nuclear weapons.

Nasrallah said Iran's response to any Israeli attack would be "lightning" quick and "great."

"It would be the golden opportunity Iran has been dreaming of for 32 years," he said, referring to the date of Iran's 1979 Revolution that led to the current clerical state. He did not mention what an attack on Iran would mean for Hezbollah.

Israel and Hezbollah fought an inconclusive monthlong war in 2006 that left 1,200 Lebanese and 160 Israelis dead. Hezbollah fired nearly 4,000 rockets at Israel during the war, and is believed to have upgraded its munitions in recent years.

Nasrallah, who fears Israeli assassination, went underground during the 2006 war and since then has rarely made any public appearances. He speaks to supporters via satellite link.

It is a sensitive time for Hezbollah, which is backed by Iran and Syria, as it faces the possibility of losing a crucial ally if Syrian rebels manage to topple President Bashar Assad.

Its reputation as a popular resistance movement has already taken a severe beating for siding with Syria against the anti-Assad uprising even after it supported Arab revolts in Egypt, Tunisia, Libya and Bahrain.

Assad's fall would be a nightmare scenario for Hezbollah. Any new regime led by the country's majority Sunni Muslims would likely be far less friendly to Shiite Muslim Hezbollah, or even outright hostile. Iran remains the group's most important patron, but Syria is a crucial supply route. Without it, Hezbollah will struggle to get money and weapons as easily.

Hezbollah has been in conflict with Israel since 1982, when the Jewish state invaded Lebanon. Israel withdrew in 2000 to a border drawn up by the U.N., but Lebanon says its southern neighbor still occupies a slice of its territory.