Iran on Saturday evening launched drones toward Israel in a retaliatory attack, the Israeli military said.

In a statement, Israel Defense Forces said that Iran launched unmanned aerial vehicles "from within its territory toward Israel." 

"The IDF is on high alert and is constantly monitoring the operational situation," the statement read. "The IDF Aerial Defense Array is on high alert, along with IAF fighter jets and Israeli Navy vessels that are on a defense mission in Israeli airspace. The IDF is monitoring all targets."

Israeli officials told CBS News it could be many hours before the drones reach Israeli airspace, which Israel closed at midnight local time. Alerts started to sound across Israel close to 2 a.m. local time, the IDF said. Alarms went off in Southern Israel, by the Dead Sea, in Jerusalem, and the Shomron area.

U.S. forces in the region have also shot down some of the Iranian-launched drones, two U.S. officials told CBS News. 

"Iran has begun an airborne attack against Israel," National Security Council spokesperson Adrienne Watson said in a statement. 

Iran's attack comes in retaliation for an April 1 Israeli strike on an Iranian consulate in Damascus, Syria, which killed seven members of the Islamic Revolutionary Guard Corps.   

President Biden had returned to the White House on Saturday in preparation for an imminent attack.  

"President Biden is being regularly updated on the situation by his national security team and will meet with them this afternoon at the White House," the statement went on. "His team is in constant communication with Israeli officials as well as other partners and allies. 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."

The U.S. is on standby for further action by Iran, and from within the region via its proxies, a U.S. official told CBS News.

The U.S. is positioned to be able to shoot down incoming drones from Iran via assets in Iraq and Syria, three U.S. officials told CBS News. The U.S. also has fighter jets that are now on standby.

The U.S. assets above are in addition to Navy destroyer USS Carney staying in the Central Mediterranean to provide additional protection if needed instead of heading West, and the destroyer USS Arleigh Burke remaining in the Eastern Mediterranean where it has been for a while. 

The U.S. preference is for the Israeli government to wait and assess the impact of the Iranian reprisal before responding to it. The U.S. preference is for a calibrated response. The expectation is that the Israelis will calibrate based on whether it successfully intercepts Iranian incoming and whether or not there are any casualties. 

Israeli Prime Minister Benjamin Netanyahu convened his war cabinet in Tel Aviv after issued a video address Saturday night, saying, "In recent years, and even more so in recent weeks, Israel has been preparing for the possibility of a direct attack from Iran. Our defense systems are deployed, we are prepared for any scenario, both in defense and attack. The State of Israel is strong, the IDF is strong, the public is strong." 

In a statement, the IRGC acknowledged that attack, saying Iran had "launched a punitive strike against the occupied territories." 

"This operation involved the use of both missiles and drones," the IRGC said. Iran's U.N. mission said the attack was in response to the strike in Syria and that, "the matter can be deemed concluded. However, should the Israeli regime make another mistake, Iran's response will be considerably more severe."

The mission added; "It is a conflict between Iran and the rogue Israeli regime, from which the U.S. MUST STAY AWAY!" 

A U.S. official in the region told CBS News that anything that passes over Jordanian territory is a problem and will be intercepted.

A U.K. official also confirmed that the British have scrambled jets from Cyprus. Britan's defense ministry later said in a statement that it had moved additional fighter jets and air refueling tankers to the region.

"These UK jets will intercept any airborne attacks within range of our existing missions, as required," the defense ministry said. 

In anticipation of the attack, earlier Saturday, the Israel Home Front Command issued guidelines limiting gathering to a maximum of 1,000 people. All schools were closed through at least Monday. People were advised to remain near safe rooms and shelters. The workweek in Israel runs from Sunday through Thursday. 

On Friday, Mr. Biden urged Iran not to move forward, saying his message to Tehran was: "Don't." Earlier in the week the U.S. sent a senior general to Israel to coordinate with the close American ally on any response it might make to an Iranian attack. 

British Prime Minister Rishi Sunak issued a statement condemning Iran's drone attack "in the strongest terms." 

Josep Borrell, European Union foreign affairs chief, said in a social media post that the EU "strongly condemns the unacceptable Iranian attack against Israel, and calling it "an unprecedented escalation and a grave threat to regional security."

Tensions in the region, however, continued to rise. Earlier Saturday, commandos from special forces unit of Iran's paramilitary Revolutionary Guard seized an Israeli-affiliated container ship near the Strait of Hormuz.

The U.S. government called on Iran to release the vessel and its international crew immediately. "Seizing a civilian vessel without provocation is a blatant violation of international law," said National Security Council spokesperson Adrienne Watson. "It must be condemned unequivocally, and we will work with our partners to hold Iran to account for its actions."

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.

Earlier in the day Lebanon launched missiles toward northern Israel. State media reported Jordan has closed its airspace "in light of the escalating risks in the region," and declared a state of emergency.

Two U.S. officials had told CBS News Friday that a major Iranian attack against Israel was expected imminently, possibly to include more than 100 drones and dozens of missiles aimed at military targets inside the country. Sources had 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 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. 

— Margaret Brennan, Debora Patta and Tucker Reals contributed to this report.