"Our enemies, particularly the Zionist regime (Israel), America and its allies, should know that any kind of threat and attack or even thinking about any (military) action will be firmly responded to," Iranian television quoted Ayatollah Ali Khamenei as saying.
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"The Revolutionary Guards and army and our nation ... will answer attacks with strong slaps and iron fists," he said.
Meanwhile, Ali Baqeri, deputy secretary of Iran's Supreme National Security Council, also warned Israel on Thursday that a military strike against Iran would create a threat to its own survival.
"If the Zionist regime allows itself such an oversight, a question of its existence will arise, not a question of its legitimacy but a question of its existence," Ali Baqeri, deputy secretary of Iran's Supreme National Security Council, said through an interpreter during a visit to Moscow.
Speculation about an imminent attack on Iran was fuelled last week when Israel test-launched a long-range missile near Tel Aviv and by comments by some Israeli officials that Tehran's nuclear program posed a "direct and heavy" threat.
Tension rose on Tuesday when the International Atomic Energy Agency reported that Iran appeared to have worked on designing an atomic bomb and may still be conducting secret research.
Iran denounced the UN inspectors' report as "unbalanced" and "politically motivated."
Israel on Wednesday called on the world to stop Iran from developing nuclear weapons while Western powers called for heavier sanctions against the Islamic Republic.
The United States and European allies say Iran is trying to build bombs under cover of a civilian nuclear program.
The major oil producer denies this, saying it needs nuclear technology to improve its electricity supply for a rapidly growing population.
Israel, which Iran refuses to recognize, sees a nuclear Iran as a threat to its existence and has said all options are on the table in confronting Tehran, including a military one. But it remains publicly committed to a world power strategy of increased diplomatic pressure and sanctions against Iran.
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