Iran ? Iran’s supreme leader warned the United States on Wednesday that his nation would hit back twice as hard if America attacked its nuclear sites.

Even as it threatened to ravage U.S. global interests, Iran sent its top nuclear official to Vienna, Austria, for talks with the U.N.’s nuclear watchdog agency ahead of a Friday Security Council deadline for Tehran to halt its uranium enrichment activities.

“The Americans should know that if they invade Iran, their interests around the world would be harmed,” supreme leader Ayatollah Ali Khamenei told workers gathered ahead of May Day, the international workers’ holiday, state television reported.

“Iran will respond twofold to any attack,” Khamenei said.

The United States repeatedly has said it has no plans to attack Iran but all options remain on the table as it pursues a diplomatic solution to Iran’s insistence on enriching uranium – a process that can produce fuel for generators or fissile material for nuclear bombs.