Israel-Gaza updates: 22, including 18 children, reported dead in Rafah strikes

The U.S. will try to help Israel intercept everything possible -- not just those that pass over U.S. ships -- despite Iran informing the U.S. that they should stay out of it, a senior U.S. official confirmed to ABC News.

In the coming hours the U.S. expects missile launches from Iran and southern Lebanon -- and maybe even the Houthis -- as well as drones, which take longer to reach target. The Iranians are trying to overwhelm the Israeli air defenses with drones and missiles coming in at different altitudes, speeds and directions -- but hitting targets at the same time, according to the official.

While the U.S. expects that most of these will be intercepted -- upwards of 85% -- the fear is that if any Israeli lives are lost, the Israeli response will be much bigger.

The target is believed to be three military bases, especially one where F-35s are kept. While these bases are relatively remote, there are towns nearby and these Iranian weapons are not completely accurate.

The U.S. has no doubt Israel will respond whether lives are lost or not. And that Iran itself will be targeted.

-ABC News' Martha Raddatz

Sources at the White House say there’s deep concern that an Iranian retaliatory strike against Israel -- and the possible Israeli response -- will widen this war.

President Joe Biden cut his weekend vacation short to consult with his national security team in preparation for this. They’ve been on high alert and knew this attack was imminent.

U.S. officials the military is prepared to provide assistance to defend Israel against attacks if needed.

The U.S. is already moving military assets to the region to deter Iran and help protect U.S. forces. Officials say they have been urging Iran against further escalation or attacks on U.S. forces. They’ve been stressing that the U.S. was not involved in the Damascus, Syria, strike earlier this month.

-ABC News' Selina Wang

Iran's leader has confirmed that a retaliatory attack on Israel has begun.

"The evil regime will be punished", Iran's Supreme Leader Ayatollah Ali Khamenei said in a statement on X.

Iranian state TV also confirmed the attack in a banner.

"The extensive drone operation of the Islamic Revolutionary Guard Corps against targets in the occupied territories has started a few minutes ago," the banner said.

A U.S. official has confirmed Israel's announcement that Iran’s retaliatory strike has begun.

President Joe Biden will meet with principals of the National Security Council to discuss events in the Middle East Saturday afternoon. The meeting will take place in the White House Situation Room, the White House confirmed.

"Iran has begun an airborne attack against Israel. President Biden is being regularly updated on the situation by his national security team," the White House said in a statement.

"This attack is likely to unfold over a number of hours. President Biden has been clear: our support for Israel's security is ironclad. The United States will stand with the people of Israel and support their defense against these threats from Iran," it added.

-ABC News' Luis Martinez and Mary Bruce