Tehran, Iran - Iran's Supreme Leader Ayatollah Ali Khamenei said Wednesday the nation would never surrender and warned the US it would face "irreparable damage" if it intervenes in support of Israel's unprecedented aggression.

The speech came six days into the conflict, with Trump demanding Iran's "unconditional surrender" while boasting the US could kill Khamenei and fuelling speculation about a possible direct, offensive intervention.

Israel's unprovoked attack began Friday, when it launched a massive bombing campaign that prompted Iran to respond with missiles and drones.

"This nation will never surrender," Khamenei said in a speech read on state TV – whose headquarters were bombed by Israel Monday.

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He called Trump's ultimatum "unacceptable" and added: "America should know that any military intervention will undoubtedly result in irreparable damage."

Khamenei, in power since 1989 and the final arbiter of all matters of state in Iran, had earlier vowed the country would show "no mercy" towards Israel's leaders.

The speech followed a night of strikes, with Israeli attacks destroying two buildings making centrifuge components for Iran's nuclear program near Tehran, according to the UN nuclear watchdog.

Centrifuges are vital for uranium enrichment, the sensitive process that can produce fuel for reactors or, in highly extended form, the core of a nuclear warhead.

Despite shrill claims from politicians, including Israeli Prime Minister Benjamin Netanyahu – who is wanted by the International Criminal Court – and Trump himself, there is no evidence Tehran was close to developing nuclear weapons. Its government has repeatedly denied having that intention.

Since Friday, at least 24 people have been killed in Israel and hundreds wounded, while Israeli strikes have killed at least 224, including military commanders, nuclear scientists, and many civilians.