Iran's Supreme Leader Ayatollah Ali Khamenei has warned the United States and Israel not to attack Iranian nuclear sites, saying his country will respond to such action with “strong slaps and iron fists.”

In a nationally-televised speech to military cadets in Tehran Thursday, Ayatollah Khamenei said Iran does not intend to start a conflict. But he said Iran will strike back.

The comments were the strongest yet by Iran since Israeli officials began making references to potential military action against the Iranian nuclear program in recent days. Israel and the West suspect Iran is trying to develop nuclear weapons under cover of a civilian nuclear program, a charge Iran denies.

The International Atomic Energy Agency released a report Tuesday saying it has “credible” evidence of Iranian efforts to design a nuclear weapon. Iranian President Mahmoud Ahmadinejad rejected the accusation and vowed his country will not retreat “one iota” from its nuclear activities. Iran says those activities are aimed at the peaceful generation of energy.

Israel responded to the IAEA report by calling on the international community to stop Iran from gaining nuclear weapons and endangering world peace. Israel is widely believed to have the only nuclear arsenal in the Middle East. It sees a nuclear-armed Iran as a threat to its existence.

Britain, France and Germany warned that Iran faces additional sanctions if it refuses to address international concerns about its nuclear work. But, China's foreign ministry said Thursday new sanctions will not “fundamentally” resolve the Iranian nuclear dispute. It called for more dialogue between the parties.

The U.N. Security Council has passed four sets of sanctions on Iran for refusing to stop nuclear activities that have both civilian and military uses. Permanent Council members China and Russia have shown little willingness to impose more.