Sheik Hassan Nasrallah, leader of Hezbollah, told thousands of supporters on Tuesday night by video link that in case of an Israeli attack on Iran, his leadership would make a decision about a response. Hezbollah is funded by Iran.
The United States Europe and Israel charge Iran is trying to build nuclear weapons. Iran says its nuclear program is for peaceful purposes.
Israel considers Iran to be its most dangerous enemy and has vowed to prevent it from acquiring nuclear weapons. Many fear that if Israel attacks Iran, it would ignite a Mideast war, starting with Hezbollah retaliation.
"There is speculation about what wound happen if Israel bombed Iran's nuclear facilities," Nasrallah said. "I tell you that the Iranian leadership will not ask Hezbollah to do anything. On that day, we will sit, think and decide what we will do."
Hezbollah holds the balance of power in Lebanon's coalition government.
The Iran-backed group has been in conflict with Israel since 1982, when Israel invaded Lebanon. Israel pulled out in 2000 according to a border drawn by the U.N., but Lebanon says Israel is still occupying some territory.
In 2006, Hezbollah and Israel fought a 34-day war the killed about 1,200 people in Lebanon and 160 in Israel. Hezbollah fired nearly 4,000 rockets at Israel. Nasrallah also denied U.S. charges that his group is involved in drug smuggling and money laundering.
In December, the Obama administration imposed sanctions on two Lebanese-Colombian men and dozens of companies they run for allegedly laundering money on behalf of Mexican and Colombian drug cartels.
U.S. officials said that one of the companies was tied to an alleged drug kingpin who has been accused of running a money-laundering scheme on behalf of Hezbollah.
U.S. federal authorities said Lebanese financial institutions wired more than $300 million to the United States in a laundering scheme they said used the U.S. financial system to benefit Hezbollah.
The U.S. accusations came after an indictment in federal court in Virginia accused fugitive Ayman Joumaa of leading a drug conspiracy that provided income for Hezbollah, which has been designated by the U.S. State Department as a terrorist organization since 1997.
"We do not deal in drugs because it is religiously prohibited," Nasrallah said.
The black-turbaned Hezbollah leader said charges about drug dealing are "lies and fabrications, and the same regarding money laundering." He said his group does not run businesses in Lebanon or abroad.
He said Hezbollah that Iran funds his group and they are not short of money. Iran reportedly has given Hezbollah billions of dollars since 1982.