Iran's Supreme Leader Ayatollah Ali Khamenei called on Muslims on Saturday "to stand by the people of Lebanon and the proud Hezbollah with whatever means they have and assist them in confronting the ... wicked regime (of Israel)."
In a statement after the Israeli army said it had killed Hezbollah leader Hassan Nasrallah, Khamenei said: "The fate of this region will be determined by the forces of resistance, with Hezbollah at the forefront," state media reported.
He has been transferred to a secure location inside the country with heightened security measures in place, two regional officials briefed by Tehran told Reuters.
The sources said Iran was in constant contact with Lebanon's Hezbollah and other regional proxy groups to determine the next step after Israel announced that it had killed Hezbollah terror chief Hassan Nasrallah in a strike on south Beirut on Friday.
Nasrallah was killed alongside Hezbollah's commander of the southern front, Ali Karaki, and a host of other senior Hezbollah members in a strike on Hezbollah's military headquarters in the Lebanese capital.
Khameini in hiding: Decision comes after emergency meeting
On Friday, Khameini held an emergency meeting with top advisors in Tehran, as per the New York Times citing Iranian sources.
Iranian Foreign Minister Abbas Araqchi accused Israel of using several US "bunker buster" bombs to strike Beirut on Friday.
"Just this morning, the Israeli regime used several 5,000-pound bunker busters that had been gifted to them by the United States to hit residential areas in Beirut," he told a UN Security Council meeting on the Middle East.
Further, US President Joe Biden directed the Pentagon to "assess and adjust as necessary US force posture" in the Middle East, according to the White House.
"He has also directed his team to ensure that US embassies in the region take all protective measures as appropriate," a statement read. The White House said Biden was briefed "several times" on Friday about the Middle East. An official added that Vice President Kamala Harris was also briefed.