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.

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.

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.

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.

Iranian officials have repeatedly denied seeking a nuclear weapon. The United States and Iran have held five rounds of negotiations toward achieving an agreement that would limit Iran's nuclear activities in exchange for sanctions relief, with a sixth round of talks scheduled for Sunday.

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

The two sides were scheduled to meet for a sixth round of negotiations on Sunday. Oman, which was set to host the talks, said on Saturday that this round had been canceled.

Tehran said American bases in the Middle East could face reprisals if the U.S. helped repel Iranian attacks on Israel.