Israeli strikes sealed off Lebanon's main border crossing with Syria early on Friday, hours after an intense Israeli attack on Beirut's southern suburbs that is thought to have targeted the heir apparent to Hezbollah's slain secretary general.

The strikes added to fears inside Lebanon that Israel's targeting of Iran-backed Lebanese Hezbollah militants will bring an all-out conflict, with Israel also poised to respond to Tuesday's Iranian missile barrage on its territory.

Iran's Supreme Leader Ayatollah Ali Khamenei, in a rare appearance leading Friday prayers in Tehran, urged Muslim nations from Afghanistan to Yemen join what he called defence against Israel. He said the missile attack was the minimum punishment for what he characterized as Israel's "crimes."

"The brilliant action of our armed forces a couple of nights ago was completely legal and legitimate," said Khamenei.

The semi-official Iranian news agency SNN quoted Revolutionary Guards deputy commander Ali Fadavi as saying on Friday that if Israel attacks, Tehran would target Israeli energy and gas installations.

Lebanese Transport Minister Ali Hamieh told Reuters Friday's strike on the Syrian border hit inside Lebanese territory near the crossing, creating a four-metre wide crater.

The Israeli Defence Forces (IDF) said it had struck an underground tunnel crossing from the Lebanese border into Syria that enabled the transfer and storage of large quantities of weapons.

According to Lebanese government statistics, more than 300,000 people — a vast majority of them Syrian — had crossed from Lebanon into Syria over the last 10 days to escape escalating Israeli bombardment.

'Alive but not alive'

Huge explosions shook the sky in the vicinity of Beirut's main airport in the early hours of Friday, and Lebanese civilians said they were living in constant fear.

"It's like you're alive but not alive. We're alive but don't know for how long, we're alive but don't know when the rockets will hit you and your family," said Nouhad Chaib, a 40-year-old man already displaced from the south.

WATCH | Lebanon's hospitals filled with war casualties:

Lebanese civilian casualties soar as Israel-Hezbollah fighting intensifies

Hundreds of thousands of Lebanese civilians are desperately trying to find shelter from Israeli airstrikes as fighting between Hezbollah and IDF forces intensifies, with hospitals filling up as the death toll climbs.

The Israeli military on Friday told the residents of over 20 southern towns in Lebanon to leave immediately, spokesperson Avichay Adraee said on X as Israel pressed ahead with its incursions in the region. Nearly 90 villages in the south have been told to evacuate so far, as well as parts of Beirut's southern suburbs.

An Israeli strike on the southern Lebanese town of Marjayoun on Friday hit around five metres from the entrance of its main hospital and medical staff have decided to temporarily evacuate, the hospital director Mounes Klakesh told Reuters.

"No one was wounded from the medical staff but we have decided to evacuate temporarily until the security situation becomes clearer," Klakesh said.

The southern suburb of Dahiye, a Hezbollah stronghold, came under renewed strikes near midnight on Thursday after Israel ordered people to leave their homes in some areas, residents and security sources said.

The air raids targeted Hezbollah official Hashem Safieddine, rumoured successor to its assassinated leader Hassan Nasrallah, in an underground bunker, Axios reporter Barak Ravid said on X, citing three Israeli officials.

Safieddine's fate was not clear, he said.

Israel's military made no comment on Safieddine's fate.

Lt.-Col. Nadav Shoshani said the military was still assessing the damage caused by the midnight airstrikes, which he said targeted Hezbollah's intelligence headquarters.

Shoshani estimated that the Israeli military had killed around 250 Hezbollah fighters, including a number of battalion and company commanders, since the start of its ground operation in Lebanon earlier this week.

Israel claims killing of another militant

The Israeli military said on Friday that it had killed the head of Hezbollah's communication networks, Mohammad Rashid Sakafi, by conducting a "precise, intelligence-based strike" in Beirut on Thursday.

Hezbollah announced several attacks on Friday at positions within Israel, including a salvo of missiles on Israel's Ilania base.

WATCH l Washington has leverage if Israel wants to strike in Iran, analyst says:

Iran a challenging target for Israel alone, analyst says

Ian Bremmer, a Middle East analyst and founder of the Eurasia Group, says vast distances and hardened targets make Iran a country that Israel would have a difficult time striking effectively on its own.

Khamenei's comments came as Iran Foreign Minister Abbas Araqchi landed in Beirut on Friday, according to Lebanese state media. He is set to meet Lebanon's caretaker Prime Minister Najib Mikati and Speaker of parliament Nabih Berri, who is a close ally of Hezbollah.

While the United States, the European Union, and other allies have called for an immediate 21-day ceasefire in the Israel-Lebanon conflict, U.S. President Joe Biden said the White House was discussing with Israel its options for responding to Tehran's assault, which included Israel striking Iran's oil facilities.

Biden's comments contributed to a five per cent rise Thursday in U.S. crude oil prices.