Iran threatened US bases in the Middle East after massive air strikes that the US claimed had destroyed Iran's nuclear programme, though some officials cautioned that the extent of damage was unclear.
International concern intensified that the unprecedented US attacks would deepen conflict in the Middle East after Israel launched an aerial war against Iran earlier this month.
Ali Akbar Velayati, an advisor to Iran's supreme leader Ayatollah Ali Khamenei, said bases used by US forces could be attacked in retaliation.
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"Any country in the region or elsewhere that is used by American forces to strike Iran will be considered a legitimate target for our armed forces," he said in a message carried by the official IRNA news agency.
"America has attacked the heart of the Islamic world and must await irreparable consequences."
US President Donald Trump urged Iran to end the conflict after he launched surprise "bunker buster" attacks on a key underground uranium enrichment site at Fordo, along with nuclear facilities in Isfahan and Natanz.
"We had a spectacular military success yesterday, taking the 'bomb' right out of their hands (and they would use it if they could!)" he claimed on his social media site.
US Defence Secretary Pete Hegseth claimed in a Pentagon press briefing that Iran's nuclear programme was "devastated," adding that the operation "did not target Iranian troops or the Iranian people".
Standing beside Mr Hegseth, top US general Dan Caine said "it would be way too early for me to comment on what may or may not still be there."
"Initial battle damage assessments indicate that all three sites sustained extremely severe damage and destruction."
Tehran protests
As Iran's leaders struck a defiant tone, President Masoud Pezeshkian also vowed that the United States would "receive a response" to the attacks.
People gathered in the centre of Tehran to protest against US and Israeli attacks, waving flags and chanting slogans.
In an address to the nation hours after the attack, Mr Trump had claimed total success for the operation, and Vice President JD Vance followed up on this morning.
"We know that we set the Iranian nuclear programme back substantially last night," he told ABC.
However, he also suggested Iran still had its highly enriched uranium.
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Watch: Trump says US attack 'obliterated' Iran nuclear sites
"We're going to work in the coming weeks to ensure that we do something with that fuel," he said.
"They no longer have the capacity to turn that stockpile of highly enriched uranium to weapons grade uranium."
Another Ayatollah Khamenei advisor, Ali Shamkhani, said in a post on X, formerly Twitter, that "even if nuclear sites are destroyed, game isn't over, enriched materials, indigenous knowledge, political will remain."
Rafael Grossi, the head of the UN's International Atomic Energy Agency (IAEA), told CNN there were clear signs of the hit on Fordo.
But no one knows "how much it has been damaged", he said.
The IAEA said it had not detected any increase in radiation at the nuclear sites and Iran said there were no signs of contamination.
Israeli Prime Minister Benjamin Netanyahu prayed for Mr Trump at the Western Wall in Jerusalem, after claiming the strikes would "change history".
Retaliation risk
The Israeli military was also checking the results of the US raid on the deeply buried nuclear facility in Fordo, with a spokesman saying it was uncertain if Iran had already removed enriched uranium from the site.
The main US strike group was seven B-2 Spirit stealth bombers that flew 18 hours from the American mainland to Iran, Mr Caine said.
In response to the attack, which used over a dozen massive "bunker buster" bombs, Iran's armed forces said they targeted multiple sites in Israel including Ben Gurion airport, the country's main international gateway near Tel Aviv.
Israeli rescuers said at least 23 people were wounded.
In Jerusalem, Claudio Hazan, a 62-year-old software engineer, said he hoped the US intervention would hasten an end to the Iran-Israel war.
"Israel by itself would not stop... and it would take longer," he said.
At least nine members of the Revolutionary Guards were killed in Israeli attacks on central Iran, local media reported, as fighting between the two foes continued.
Israeli attacks on Iran have killed more than 400 people since they began, Iran's health ministry said.
Iran's attacks on Israel have killed 24 people, according to official figures.
The United Arab Emirates, Qatar and Oman, which had been mediating Iran-US nuclear talks, criticised the US strikes and called for de-escalation.
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The leaders of France, Germany and the UK urged Iran "not to take any further action that could destabilize the region" and UN chief António Guterres warned of a cycle of retaliation.
Iran's Houthi allies in Yemen repeated their threat to resume attacks in the Red Sea if the US joined the war, saying they were ready to target US ships.
The US president had stepped up his rhetoric against Iran since Israel first attacked the country on 13 June, repeating his insistence it could never have nuclear weapons.
However, Israel is widely acknowledged to be the only country in the Middle East in possession of nuclear weapons, a claim which it denies.
