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

White House National Security Council spokesman John Kirby told ABC's "Good Morning America" Sunday that any response to Iran's Saturday night attack is up to Israeli forces.

But Kirby stressed that President Joe Biden does not want the situation to escalate or have the U.S. drawn further into any conflict.

When asked by ABC News' Whit Johnson about a response from Israel, Kirby said that it is up to the Israeli government to decide how to respond. He added that "the damage was extremely light," and the defenses in place proved Israel can defend itself.

"I won't speak for the Israelis. It's going to be up to them to decide whether and how they'll respond to this," Kirby said. "They showed last night, an incredible military capability on their own, but certainly in concert with friends."

When pressed about reports that Biden told Israeli Prime Minister Benjamin Netanyahu the U.S. would oppose a counterattack on Iran, Kirby said it was "not an accurate reading" of the president's message. He emphasized that the White House does not "want to see the situation escalate further."

"The president understands the prime minister runs a government for a sovereign state of Israel, and that they'll decide whether and how they're going to respond to what Iran did last night," Kirby said.

Kirby reiterated the White House's message that the president will do "whatever he has to protect our troops and our facilities, our people in the region." He said there is no current threat to U.S. service members and facilities, noting that more U.S. forces moved into the region after Oct. 7 to help defend Israel and shipping channels in the Red Sea.

"We're going to obviously be vigilant to any potential threat to our forces in the region," Kirby said. "But the president has been clear we are not looking for a wider war. We're not looking for a second front or a third front. We're not looking to see escalation and we're certainly not looking for a war with Iran."

Kirby added that good intelligence about the attack helped the U.S. and Israel prepare.

"We've been watching as closely as we could the intelligence picture, we had a pretty good indication of the size and the scale and the scope of what Iran was planning," Kirby said. "That is why, because we had a good sense of what they were going to do and with how much, we were able to really help Israel knock down almost everything that Iran threw at them."