Israel-Iran updates: Iran launches more missiles at Israel, sirens sound in Tel Aviv and Jerusalem

The head of Iran's Islamic Revolutionary Guard Corps Gen. Hossein Salami was killed in Israeli strikes on Iran, Iranian state TV reports.

Also killed was Dr. Fereydoun Abbasi, former head of the Atomic Energy Organization, according to Iranian state TV.

Israel has not confirmed the deaths.

A senior security official said, "There is a growing likelihood that the Iranian General Staff, including the Iranian Chief of Staff, and senior nuclear scientists were eliminated in the opening blow."

During a televised address, Israel Defense Forces Chief Eyal Zamir said the military was calling up "tens of thousands of soldiers" and was "prepared across all borders." The official went on to say, "I warn that anyone who will try to challenge us, will pay a heavy price."

Following Israel's series of strikes on Iran, the U.S. Embassy in Israel has directed all American government employees and their family members to shelter in place until further notice.

"The U.S. Embassy reminds U.S. citizens of the continued need for caution and increased personal security awareness – including knowing the location of the nearest shelter in the event of a red alert as security incidents, including mortar, rocket, and missile fire, and unmanned aircraft system (UAS) intrusions, often take place without any warning," the embassy said in a statement, adding, "The security environment is complex and can change quickly."

A senior Israeli security official told ABC News that the Iranian General Staff and senior nuclear scientists were likely killed in the first of the series of Israeli strikes on Iran's nuclear targets.

"There is a growing likelihood that the Iranian General Staff, including the Iranian Chief of Staff, and senior nuclear scientists were eliminated in the opening blow," the official said.

-ABC News' Dana Hughes