The United States is the main international backer of Israel, Iran's biggest regional foe. It is also a close ally of Sunni Gulf monarchies which for years pursued their own rivalry with the Islamic Republic, seeing it as their main regional threat.

Israel's operation against Iran is projected to last "weeks, not days," with implicit US approval, according to White House and Israeli officials.

Iran activated its air defences in several regions on Saturday and Israel told its citizens to take shelter ahead of a fresh barrage of missiles, as the arch-foes exchanged massive strikes in their fiercest direct confrontation in history.

Trump told the Wall Street Journal that he knew all about Israel’s plan ahead of time, that he wasn’t given a mere “heads-up.”

The United States has pulled some diplomatic staff and military families out of the Middle East, citing unspecified regional security risks.

Countries trade strikes as tensions grow over larger Middle East conflict - Iranian missiles struck an apartment building in the Galilee region, killing four people, as the countries exchanged fire

Iranian officials said on Friday that the United States "shall be held fully accountable" for its support of Israel after the nation took out key Iranian nuclear facilities and killed government leaders in a series of air strikes this week.

Interviews with half a dozen senior Iranian officials show that they were not expecting Israel to strike before another round of talks.