“Iran made a big mistake – and it will pay for it,” he said. “Whoever attacks us, we will attack them.”

The US may have more leverage with Israel in shaping its response to Iran than it has had in Gaza and Lebanon.

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Together with Britain and other allies, the US “actively supported” Israel in tracking and shooting down Iran’s missiles on Wednesday (AEST), just as it did in April when Iran directly attacked Israel for the first time.

It is also widely believed that Israel would need assistance from the US to effectively destroy Iran’s underground nuclear sites, a move that could provoke an all-out regional war.

“We’ll be discussing with the Israelis what they’re going to do, but all seven of us [G7 nations] agree that they have a right to respond, but they should respond proportionally,” Biden said.

Earlier Jake Sullivan, the US National security adviser, promised that Iran would face “severe consequences” and that the US would work with Israel to ensure as much, but did not elaborate.

Israel continued to fight on several fronts on Thursday with clashes in Lebanon, Gaza, the West Bank and Syria.

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At the same time, more than 240 rockets were launched by Hezbollah at northern Israel, according to the military, as Israel ramped up its operations against the Iranian proxy on the ground in southern Lebanon.

Hezbollah said it had engaged Israeli troops in at least three different locations along the border, killing and wounding several.

Later, Israel’s military confirmed the deaths of eight soldiers killed in southern Lebanon, all in their early 20s.

“I wish to extend my deepest condolences to the families of our heroes who fell today in Lebanon,” said Netanyahu on the eve of the Jewish new year. “May God avenge them. May their memory be a blessing.”

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The Israeli response to Iran could also include targeted assassinations, like that which killed Ismail Haniyeh in Tehran in July, and Hassan Nasrallah in Lebanon, Israeli sources told Axios.

However, the country’s war cabinet had not decided what form the response would take and wanted to first discuss options with the US, it added.

“We have a big question mark about how the Iranians are going to respond to an attack, but we take into consideration the possibility that they would go all in, which will be a whole different ball game,” an Israeli official told Axios.

Iran appeared to have used new faster ballistic missiles against Israel in its barrage and it was not clear how many had evaded the country’s air defence systems.

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The IDF admitted several air force bases were damaged in the attack but that no weapons, soldiers or aircraft were hit, and they had not been stopped from operating “at any stage”.

Footage showed missiles and explosions lighting up the night sky, arriving far faster and thicker than in the previous Iranian strike in April.

The IDF would not reveal what the interception rate was, saying this would aid Iran. It did say that the country’s air defences had “operated impressively, with high rates of interception”.

It also emerged that Iran’s Supreme Leader Ayatollah Ali Khamenei warned Hezbollah leader Hassan Nasrallah to flee Lebanon days before he was killed in an Israeli strike.

Three Iranian sources told Reuters that Khamenei was deeply worried about Israeli infiltration of senior government ranks in Tehran, citing three Iranian sources.

Khamenei, who has remained in a secure location inside Iran since Saturday, personally ordered the barrage of more than 180 missiles to be fired at Israel, according to reports.

Pro-Iranian media claimed that the Islamic Revolutionary Guard Corps (IRGC) had “acquired new technologies and equipment” to surprise Israel with if it is attacked.

“The IRGC’s several-week delay in retaliating against Zionist terrorists was due to the completion and development of certain undisclosed technologies that place Iran in a far superior position in its confrontation with the Zionists,” reports said.

‘Knife’s edge’

The Biden administration will now focus its engagements with the Israelis on aligning its perspectives on any potential response to the Iranian attack on Israel, said Kurt Campbell, the US Deputy Secretary of State, describing the situation in the Middle East as “a moment of peril”.

“I think we recognise as important as a response of some kind should be, there is a recognition that the region is really on a knife’s edge,” said Campbell, speaking at a virtual event by Carnegie Endowment, a Washington-based think tank.

Assaf Orion, a retired Israeli brigadier general and fellow at the Washington Institute, said he thought further escalation between Israel and Iran was now inevitable but that it could still be contained short of all-out war.

“We haven’t seen the end of it, and we don’t see any specific diplomatic off ramp, so I think we have some way to go,” he said.

The Telegraph, London