President Donald Trump has said that the US military struck three sites in Iran, directly joining Israel's effort to decapitate the country's nuclear programme in a risky gambit to weaken a longtime foe amid Tehran's threat of reprisals that could spark a wider regional conflict.

The decision to directly involve the US comes after more than a week of strikes by Israel on Iran that have moved to systematically eradicate the country's air defences and offensive missile capabilities, while damaging its nuclear enrichment facilities.

But US and Israeli officials have said that American stealth bombers and a 30,000-lb bunker buster bomb they alone can carry offered the best chance of destroying heavily-fortified sites connected to the Iranian nuclear programme buried deep underground.