Washington DC - President Donald Trump said Sunday the US would hit Iran with "force that has never been seen before" if the country retaliated against US and Israeli strikes that killed Supreme Leader Ayatollah Ali Khamenei.

Iran indicated it was "going to hit very hard today, harder than they have ever hit before," wrote Trump, who has vowed to seek regime change in the Islamic republic, on his Truth Social platform early Sunday.

"THEY BETTER NOT DO THAT, HOWEVER, BECAUSE IF THEY DO, WE WILL HIT THEM WITH A FORCE THAT HAS NEVER BEEN SEEN BEFORE!" he said.

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The shocking bombing campaign by the US and Israel – which followed a massive military buildup in the Middle East unseen since the 2003 invasion of Iraq – killed Khamenei, Iranian state television confirmed Sunday.

Iran has since retaliated, with reported Iranian strikes across the Gulf, including in the Emirati capital Abu Dhabi and near regional US military bases.

Trump has said the US bombing will continue "as long as necessary" and noted Khamenei's killing gives Iranians their "greatest chance" for taking power from the current regime.