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 Hezbollah militants will bring an all-out conflict, with Israel also poised to respond to Tuesday's Iranian missile barrage on its territory. U.S. President Joe Biden said on Thursday Israel's response could include a strike on Iran's oil facilities.

Lebanese Transport Minister Ali Hamieh said Friday's strike on the Syrian border hit inside Lebanese territory near the crossing, creating a four-metre (12 feet) wide crater.

The Israeli Defense Forces (IDF) had accused Hezbollah on Thursday of using the crossing with Syria to transport military equipment into Lebanon. Meanwhile, Hezbollah said on Friday it had targeted Israeli troops in the Lebanon border area.

"The IDF will not allow the smuggling of these weapons and will not hesitate to act if forced to do so, as it has done throughout this war," IDF spokesman Avichay Adraee said on X.

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 ten days to escape escalating Israeli bombardment.

The southern suburb of Dahiye, a stronghold of Hezbollah, 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 declined to comment and Hezbollah made no comment on Safieddine's fate.

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.

A UN refugee agency official said most of Lebanon's nearly 900 shelters were full and that people fleeing Israeli strikes were increasingly sleeping out in the open.

"Most of the nearly 900 government established collective shelters in Lebanon have no more capacity," UNHCR's Rula Amin told a Geneva press briefing. "With the onset of winter, UNHCR is concerned that conditions for those affected by the escalating conflict in other will only worsen," she added.

"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.

The Israeli military on Friday told the residents of over 20 southern towns in Lebanon to evacuate 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.

Three Hezbollah-linked rescue workers were wounded by a strike in a southern suburb, a Lebanese security source told Reuters.

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

Iran's 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.

U.S. President Joe Biden said he did not believe there is going to be an "all-out war" in the Middle-East, as Israel weighs options for retaliation but that more needed to be done to prevent one.

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

Israeli Prime Minister Benjamin Netanyahu has vowed Iran will pay for Tuesday's missile attack and Washington has said it would work with its long-time ally to ensure Iran faced "severe consequences".

Iranian President Masoud Pezeshkian, speaking in Doha, said on Thursday that Tehran would be ready to respond.

"Any type of military attack, terrorist act or crossing our red lines will be met with a decisive response by our armed forces," he said.

Britain said it would provide an additional £10million ($13m) of humanitarian support to Lebanon to help the country deal with the mass displacement of people and the growing number of civilian casualties.

Supreme leader Ayatollah Ali Khamenei had a rifle by his side as he addressed worshippers in Tehran in a rare public sermon Friday, days after Iran's missile attack on Israel. 

Khamenei delivered the sermon - his first in nearly five years - to thousands of worshippers carrying portraits of slain leaders of Iran's "axis of resistance" against Israel and the U.S.

Khamenei on Friday defended Iran's missile attack on Israel as "legal and legitimate".  He claimed the missile attack was the "minimum punishment" for Israel and said Israel "cannot seriously harm" Hezbollah and Hamas, adding Hezbollah's fight was a "vital service to entire region".

A source close to Hezbollah said the group's slain chief Hassan Nasrallah has been temporarily buried in a secret location fearing Israel would target a large funeral.

"Hassan Nasrallah has been temporarily buried, until the circumstances allow for a public funeral," the source said, after an Israeli strike killed the leader last week.

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

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