Update: Iran launched drone attacks against Israel on Saturday. Read CBS News’ latest coverage here.

Tel Aviv — Israel is bracing for a worst-case scenario that U.S. officials believe could materialize within just hours — the possibility of a direct attack on Israeli soil by Iran in retaliation for a strike almost two weeks ago that killed seven Iranian military officers. Iran has vowed to take revenge for Israel killing its commanders, who were hit by an April 1 strike on the Iranian embassy in Syria’s capital.

Two U.S. officials told CBS News that a major Iranian attack against Israel was expected as soon as Friday, possibly to include more than 100 drones and dozens of missiles aimed at military targets inside the country. Sources have told CBS News the retaliation could include attacks carried out both by Iranian forces, and proxy groups around the region that it has been funneling additional arms to for weeks.  

The officials said it would be challenging for the Israelis to defend against an attack of that magnitude, and while they held out the possibility that the Iranians could opt for a smaller-scale attack to avoid a dramatic escalation, their retaliation was believed to be imminent. 

Asked Friday how imminent he believes an attack is, President Biden responded, “I don’t want to get into secure information, but my expectation is sooner than later.” The president urged Iran not to move forward, saying his message to Tehran was: “Don’t.”

Tehran has not indicated publicly how or when it will return fire, so it’s unclear how far Iran’s leaders will go. If they decide to carry out a direct attack on Israel, there’s fear it could blow Israel’s ongoing war against Iranian ally Hamas up into a much wider regional conflict.

With the Iranian retaliation expected at any time, the U.S. State Department on Thursday warned Americans in Israel not to travel outside major cities, which are better protected from incoming rocket fire by the country’s Iron Dome missile defense system. The latest guidance noted that travel by U.S. government employees in Israel could be further restricted with little notice as things develop in the tinderbox region.

“Whoever harms us, we will harm them,” Prime Minister Benjamin Netanyahu vowed Thursday as he visited troops at an Israel Defense Forces airbase. “We are prepared … both defensively and offensively.”

On Saturday, all U.S. embassies in the Middle East were put on high alert and required to hold emergency action committee meetings. Diplomats in Lebanon and Israel were specifically told not to travel to certain areas within those countries.

Sima Shine, a security expert and former official with Israel’s national intelligence agency Mossad, told CBS News it was a dangerous moment for the region, and the “most worried” she has been. She said anxiety over an all-out war was likely just as high “on both sides, in Israel and in Iran.”

If Iran does choose to strike Israel directly, it could involve a complex missile and drone attack similar to the one Iranian forces launched against a Saudi oil facility in 2019.

“They will try to do it on the military or some military asset,” Shine predicted. “But the question will be the damage. If there would be many injured people, killed or injured … I think it has the potential for a huge escalation.”

Shine stressed, however, that she still believes neither side actually wants a regional conflict.

U.S. “really trying to avoid war”

The U.S. sent a senior general to Israel this week to coordinate with the close American ally on any response it might make to an Iranian attack. Speaking Friday on “CBS Mornings,” America’s top military officer said, “we’re really trying to avoid war.”

“This is part of the dialogue that I have with my counterparts within the region, to include the Israeli chief of defense, who I talked to yesterday,” said Joint Chiefs chairman Gen. Charles Q. Brown, Jr., adding that the U.S. military was “doing things not only to prevent a war, but at the same time, one of my primary things is to make sure all the forces in the region are protected.”

“My role, as the chairman of the Joint Chiefs, is to plan and prepare,” Brown said. “That’s one thing we do very well.”

Brown’s Israeli counterpart, Chief of the General Staff Lt. Gen. Herzi Halevi, “completed a comprehensive situational assessment on the readiness of the IDF for all scenarios,” Israel’s military said Friday. 

“The IDF is very strongly prepared, both offensively and defensively, against any threat,” Halevi was quoted as saying in the statement. “The IDF continues to monitor closely what is happening in Iran and different arenas, constantly preparing to deal with existing and potential threats in coordination with the United States Armed Forces.” 

The IDF said the visiting U.S. general, Central Command chief Gen. Michael Erik Kurilla, was taking part in the IDF’s situational assessment.

The dilemma for Iran, said Israeli expert Shine, is to figure out how to deliver its promised response to Israel’s attack in Syria, but in a way that does not lead to further escalation. Likewise, Shine said Israel could choose to show restraint when it responds to whatever Iran eventually does.

If either side gets the balance wrong, the consequences for the region, and even the world, could be dire.

Weijia Jiang, David Martin, Margaret Brennan and Olivia Gazis contributed reporting.