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.

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.

Iran launched dozens of ballistic missiles toward Israel on Friday night in retaliation for Israel's surprise attack early Friday. Israel struck at the heart of the Iran's nuclear program, killing several nuclear scientists as well as high-ranking military leaders, according to Israeli officials.

In sweeping attacks that started early Friday, Israel has struck at the regime in Tehran, targeting Iranian nuclear and military assets. The Israeli strikes have killed more than 70 people, including four top security chiefs, and damaged Iran’s main nuclear site at Natanz.

Israel intensified its offensive on Iran by targeting the Defence Ministry headquarters and nuclear-related facilities in Tehran, as Iran launched wave of missiles that killed at least four in northern Israel.

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.

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