Israel-Gaza-Lebanon updates: IDF orders evacuations in Beirut as huge blast hits city

Iran is expected to launch 240 to 250 missiles in two waves at four targets in Israel, including Mossad headquarters and three air bases, according to an Israeli source and a senior U.S. administration official.

Israel is not expected to launch a preemptive strike, according to the U.S. administration official. It should take the missiles less than 15 minutes to reach targets once launched.

The U.S. and Israel have all defenses activated, and Israel will “absolutely “ retaliate after it happens," the official said.

The total number of ballistic missiles is about double the number launched by Iran in April -- 99% of which were intercepted.

-ABC News' Martha Raddatz

The Israel Defense Forces issued a warning for residents of the "Dan area," referring to the greater Tel Aviv area, to stay "near a shelter until further notice."

Sirens sounded in central Israel, the IDF said in a second statement, after issuing the warning to stay in shelters. The sirens sounding in central Israel were due to launches from Lebanon, the IDF told ABC News.

"From this moment, Israelis living in central Israel are advised to stay by their safe rooms. Enter them during sirens and stay there until further notice," IDF spokesperson Daniel Hagari said in Hebrew.

-ABC News' Dana Savir, Anna Burd and Jordana Miller

The U.S. Embassy in Jerusalem issued a statement directing all government employees and their family members "to shelter in place until further notice," following reports of an imminent Iranian missile attack on Israel.

The embassy noted the "continued need for caution and increased personal security awareness as security incidents, including mortar and rocket fire and unmanned aircraft system," which it said "often take place without warning."

"The security environment remains complex and can change quickly depending on the political situation and recent events," it added.

Prime Minister Benjamin Netanyahu urged unity among Israelis in a video statement published Tuesday, as Israel Defense Forces operations in Lebanon expanded and amid fears of an imminent Iranian missile attack.

"We are in the midst of a campaign against Iran's 'Axis of Evil,'" the prime minister said. "These are days of great achievements and great challenges."

The "great achievements," Netanyahu said, included the assassination of Hezbollah leader Hassan Nasrallah in a Beirut airstrike last week, plus the defeat of an alleged Hezbollah plan to launch an infiltration attack into northern Israel.

The country still faces "big challenges," he continued.

"What I ask of you is two things: One -- to strictly obey the directives of the front-line command, it saves lives. And second -- to stand together. We will stand firm together in the testing days ahead."

"Together we will stand, together we will fight and together we will win."

-ABC News Joe Simonetti and Jordana Miller