The US has launched “major combat operations” in Iran, designed to eliminate “imminent threats” from the country’s regime, Donald Trump announced on Saturday.

The operation is “massive and ongoing”, the US president said in a video on social media, pledging to use “overwhelming strength and devastating force” to destroy Iranian missiles and ensure it cannot develop a nuclear weapon.

The Israeli prime minister, Benjamin Netanyahu, said the attacks aimed to “remove the existential threat” posed by the Iranian regime, as he urged the people of Iran to topple the government.

Shortly beforehand, Israel said it had launched “preventative” strikes on Iran.

The Israel Defense Forces said Iran had fired back a “barrage of missiles” and they were working to intercept them. Israel has closed its airspace.

Explosions were heard in Tehran, the Iranian capital, where smoke could be seen rising on the skyline, and several other cities. Iran has closed its airspace.

“The hour of your freedom is at hand,” Trump told Iranian citizens, urging them to “take over your government” once the operation was over.

Talks between the US and Iran on Tehran’s nuclear programme ended inconclusively on Friday, with a suggestion that further discussions would be held next week. Trump had said he was “not happy” with the progress of the discussions.

Iran’s supreme leader, Ayatollah Ali Khamenei, is not in Tehran, and has been transferred to a secure location, an official told Reuters on Saturday.