Hezbollah said it could kill tens of thousands of Israelis by hitting targets with precision-guided missiles in a declaration that seemed aimed at deterring Israeli strikes on Lebanon or its regional backer Iran.
Hezbollah secretary general Hassan Nasrallah said his group could turn the lives of hundreds of thousands of Israelis "to real hell" by hitting a small number of number of targets he said was "not large" - a possible reference to nuclear facilities, though Nasrallah would not go into details.
"During any stage of an attack on our country, if we are forced to use or target this type of target, to protect our people and country, we will not hesitate," he said.
Nasrallah's remarks on Friday will likely be factored into Israeli calculations ahead of any military action against Iran, which is pursuing a nuclear programme viewed as an existential threat in Israel.
Israeli Defence Minister Ehud Barak has said he believes his country would suffer up to 500 casualties in any conflict aimed at wiping out Iran's nuclear facilities - which both Israel and Western powers believe Tehran is using to develop nuclear weapons.
"We can talk about tens of thousands of dead," Nasrallah said in a speech to mark Jerusalem Day, commemorated on the last Friday of the Islamic holy month of Ramadan according to a tradition established by Iran's late Ayatollah Ruhollah Khomeini. "I tell the Israelis that you have a number of targets, not a large number … that can be hit with precision rockets … which we have.
"Hitting these targets with a small number of rockets will turn … the lives of hundreds of thousands of Zionists to real hell," he said.
Israel, thought to be the Middle East's only nuclear-armed power, has repeatedly threatened military action if diplomacy fails to resolve the Iranian nuclear issue. Iran denies seeking a bomb and says its nuclear work has only peaceful purposes.
Hezbollah is a Shiite Islamist guerilla and political movement founded with Iran's help after the Israeli invasion of Lebanon in 1982 and has deep ideological links to Iran.
The group last fought Israel in 2006 during a 34-day war in which 1,200 people in Lebanon, mostly civilians, and 160 Israelis, mostly soldiers, were killed.
Since that war, the group has a number of times suggested it had expanded its arsenal in an apparent strategy of deterrence.
Nasrallah did not say whether the precision-guided rockets he described in his speech were a new addition to the group's arsenal.
Marking Jerusalem Day in Tehran, Iranian President Mahmoud Ahmadinejad said there was no place for Israel in a future Middle East.
Nasrallah said Israel was still debating whether to attack Iran because "Iran was strong and brave … We all know that the Islamic republic's response will be very great and thunderous if it is targeted by Israel."
Nasrallah, who fears Israeli assassination, went underground during the 2006 war and now rarely makes public appearances. He speaks to supporters via satellite link.
In New York, United Nations Secretary General Ban Ki-moon said he was dismayed by the latest anti-Israel comments made by Iranian leaders. The United States and France also condemned Ahmadinejad's calling Israel a "cancerous tumour".
Ban said that "all leaders in the region should use their voices at this time to lower, rather than to escalate, tensions", said a spokesman.
Additional reporting by Associated Press