Aerial assaults meanwhile raged on, with air raid sirens sounding across Israel and AFP journalists reporting several blasts were heard over Jerusalem.

The Israeli military said it had struck missile sites in western Iran as well as "six Iranian regime airports" across the country, destroying fighter jets and helicopters.

President Donald Trump said US warplanes used "bunker buster" bombs to target sites in Fordo, Isfahan and Natanz, boasting the strikes had "obliterated" Iran's nuclear capabilities.

Other officials said it was too soon to assess the true impact on Iran's nuclear programme, which Israel and some Western states consider an existential threat.

Iranian armed forces spokesman Ebrahim Zolfaghari said on state television that the US "hostile act", following more than a week of Israeli bombardments, would "pave the way for the extension of war in the region".

"The fighters of Islam will inflict serious, unpredictable consequences on you with powerful and targeted (military) operations," he warned.

Global markets reacted nervously, with oil prices jumping more than four percent early Monday. China urged both Iran and Israel to prevent the conflict from spilling over, warning of potential economic fallout.

Oman, a key mediator in the stalled Iran-US nuclear talks, condemned the US strikes and called for calm.

Iran's foreign ministry accused Washington of betraying diplomacy.

"Future generations will not forget that the Iranians were in the middle of a diplomatic process with a country that is now at war with us," said ministry spokesman Esmaeil Baqaei.

Britain, France and Germany called on Iran "not to take any further action that could destabilise the region".

As the world awaited Iran's response, supreme leader Ayatollah Ali Khamenei called the bombing campaign Israel launched on June 13 "a big mistake".

- 'Regime change' -

US Secretary of State Marco Rubio called on China to help deter Iran from closing the Strait of Hormuz, a chokepoint for one-fifth of the world's oil supply.

With Iran threatening US bases in the region, the State Department issued a worldwide alert cautioning Americans abroad.

In central Tehran on Sunday, protesters waved flags and chanted slogans against US and Israeli attacks.

In the province of Semnan east of the capital, 46-year-old housewife Samireh said she was "truly shocked" by the strikes.

"Semnan province is very far from the nuclear facilities targeted, but I'm very concerned for the people who live near," she told AFP.

Iranian President Masoud Pezeshkian said the US strikes revealed Washington was "behind" Israel's campaign against the Islamic republic and vowed a response.

After the Pentagon stressed the goal of American intervention was not to topple the Iranian government, Trump openly toyed with the idea.

"It's not politically correct to use the term, 'Regime Change,' Trump posted on his Truth Social platform. "But if the current Iranian Regime is unable to MAKE IRAN GREAT AGAIN, why wouldn't there be a Regime change???"

Hours later he doubled down on emphasising the success of his strikes.

"Monumental Damage was done to all Nuclear sites in Iran, as shown by satellite images. Obliteration is an accurate term!" Trump wrote, without sharing the images he was referencing.

At a Pentagon press briefing earlier in the day, top US general Dan Caine said "initial battle damage assessments indicate that all three sites sustained extremely severe damage."

Israeli Prime Minister Benjamin Netanyahu, meanwhile, said his country's bombardments would "finish" once the stated objectives of destroying Iran's nuclear and missile capabilities have been achieved.

"We are very, very close to completing them," he said.

- 'Blow up' -

Israeli strikes on Iran have killed more than 400 people, Iran's health ministry said. Iran's attacks on Israel have killed 24 people, according to official figures.

Rafael Grossi, director of the International Atomic Energy Agency (IAEA), told an emergency meeting of the UN Security Council that craters were visible at the Fordo facility, but it had not been possible to assess the underground damage.

"Armed attacks on nuclear facilities should never take place," he added.

Iranian Foreign Minister Abbas Araghchi, who was due to meet with Russian President Vladimir Putin on Monday, had accused the United States of deciding to "blow up" nuclear diplomacy with its intervention in the war.

While Russia condemned the Israeli and US strikes, it has not offered military help and has downplayed its obligations under a sweeping strategic partnership agreement signed with Tehran just months ago.

On Sunday, Russia, China and Pakistan circulated a draft resolution with other Security Council members that calls for an "immediate ceasefire" in Iran.

burs-ami/dv