The Israeli military said it had begun a wave of strikes targeting Iranian launch sites, aerial defence systems and infrastructure, as well as launching air strikes on Lebanon.
In Israel, air raid sirens sounded early this morning warning of Iranian missiles, and loud blasts were heard as the missiles were intercepted shook buildings, said witnesses.
"Several projectiles that crossed from Lebanon... were intercepted," an Israeli military statement said, reporting no casualties.
Alerts were earlier activated in the town of Metula, on the border with Lebanon, where Hezbollah said its fighters had targeted troops "with a missile salvo".
Lebanese state media said Israeli strikes had hit a Beirut hotel and residential complex in Lebanon's east.
Several people were killed in the air strike on the four-storey building in Baalbek, the state news agency NNA reported.
The Israeli military has issued an evacuation warning to 16 villages in southern Lebanon, urging them to leave their homes, saying anyone near Hezbollah fighters, facilities or weapons would be putting their lives at risk.
Meanwhile, US Admiral Brad Cooper, who leads US forces in the Middle East as the head of Central Command, said the first 24 hours of the 'Operation Epic Fury' bombardment of Iran were "nearly double the scale" of the first 24 hours of the 'Shock and Awe' campaign that opened the Iraq War in 2003.
"We are seeing that Iran's ability to hit us, and our partners, is declining, while our combat power, on the other hand, is building," Adm Cooper said in a video briefing released yesterday evening.
"My overall operational assessment is that we are ahead of our game plan."
Watch: Smoke rises over Tehran after air strikes hit Iranian capital
Adm Cooper said Iran's air defences had been badly degraded, its navy had no operational vessels on key waterways after 17 were sunk, and that more than 2,000 Iranian targets had been hit.
Some 50,000 US troops were taking part in operations, and that "more capabilities" were on the way, he said.
Iran said deaths from the attacks had reached 787 by yesterday, including 165 girls killed on the war's first day when their school was bombed, the highest toll among several civilian sites reported to have been hit.
Watch: Donald Trump says 'everything's been knocked out' in Iran
US President Donald Trump has sought to justify the assault on Iran, saying he had ordered the campaign because he had "a feeling" Iran would attack after negotiations over its nuclear programme stalled.
Iran has called the war an unprovoked attack.
"We have told the enemy that if you try to harm our main centres, we will hit all economic centres in the region," Revolutionary Guards adviser Ebrahim Jabari said in Iranian media.
Iran has fired missiles and drones at neighbouring Arab states that host US bases, and strangled shipping through the Strait of Hormuz, where a fifth of the world's supplies of oil and liquefied natural gas travel past its coast.
Iran-backed Hezbollah said it had targeted northern Israel's Haifa naval base yesterday in response to the ongoing Israeli strikes, which began after an initial attack by Hezbollah in response to the killing of Iranian supreme leader Ayatollah Ali Khamenei during the US-Israeli strikes on Saturday.
Israel has carried out successive air raids on the south of the country as well as some suburbs of the capital Beirut where Hezbollah holds sway.
But according to the NNA, some of the attacks this morning hit areas outside of Hezbollah's traditional strongholds.
The NNA said that "an Israeli air strike targeted a hotel in Hazmieh", in the first reported attack on the predominantly Christian area in Beirut's suburbs.
Israeli strikes since Monday have killed at least 50 people in Lebanon, according to the health ministry.
Mr Trump has said the US navy would escort oil tankers through the Strait of Hormuz if needed amid the Iran war and ordered Washington to provide insurance for shipping.
"If necessary, the United States Navy will begin escorting tankers through the Strait of Hormuz, as soon as possible. No matter what, the United States will ensure the FREE FLOW of ENERGY to the WORLD," he said on his Truth Social platform.
In Iran, a ceremony will be held later today at Tehran's Imam Khomeini prayer ground as Iranians are to bid farewell to Khamenei in a ceremony.
The ceremony will last for three days and the funeral procession will be announced when it is finalised, state media reported.
