IRAN has vowed to seek revenge on Israel after it launched three waves of airstrikes into the country overnight - following a series of attacks from Tehran over the past year.
Dubbing it a violation of international law, Iran said it is "entitled and obligated to defend itself against external aggressive acts".
Israel's military operation - dubbed "Days of Repentance" - came as the war-torn region has been bracing for an Israeli response to Iran's drone attack on October 1.
Hamas - Iran's terror puppet group based in Gaza - slammed the Israeli strikes as an "escalation", calling it a "violation of Iran's sovereignty".
Meanwhile Iran's most powerful proxy Hezbollah, operating out of Lebanon, said today that it launched a drone attack against Israel's Tel Nof airbase, south of Tel Aviv, in response to last night's attack.
It also claimed to have targeted an intelligence base in the northern city of Safed with a rocket barrage.
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Israel has not verified the group's claims.
Iranian military sources have said four soldiers were killed in the overnight attack with the figure "expected to rise further", state news agency Tasnim News reports.
It also said others left wounded are in "critical" condition.
The Israel Defence Forces (IDF) announced on Saturday that their revenge blitz against Iranian military bases had finished, and warned that any response from Iran would force them to "pay a heavy price".
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IDF spokesperson Daniel Hagari said Israel was "conducting precise strikes on military targets in Iran" in response to "relentless" attacks by Iran and its proxies since October 7.
He added: "If the regime in Iran were to make the mistake of beginning a new round of escalation, we will be obligated to respond.
"Our message is clear: All those who threaten the State of Israel and seek to drag the region into a wider escalation will pay a heavy price."
Iranian state TV confirmed on Saturday that several strong explosions were heard around the capital and in the more remote rural regions, beginning at around 2am local time.
Two Israeli officials told The New York Times that some 20 different sites were hit in the overnight attack.
The first wave of attacks focused on Iranian air defence systems, with the second and third targeting missile and drone bases and production sites, Israeli and American officials told Axios.
Israel reportedly sent a message of warning to Iran on Friday ahead of the retaliatory airstrikes - warning them not to respond, Axios reports.
Sources said it was an attempt to limit the possibility of a wider regional conflict after the strikes.
Prime Minister Keir Starmer, speaking from Samoa, said he is monitoring the situation in the Middle East "closely".
He urged "restraint" from both sides, adding: "Iran should not respond."
"We will continue to work with allies to de-escalate the situation across the region," he said.
Witnesses in Tehran confirmed hearing loud explosions during the early morning blitz.
"It was so loud and the sky became red," said an Iranian resident in Tehran, who asked not to be named, reports.
Iranian authorities revealed that at an attack on one of their police convoys in the southern region of Sistan and Baluchestan killed at least 10 officers on Saturday.
Authorities identified no immediate suspects for the attack, and no group has claimed responsibility.
Last night's blitz marked the first time Iran has been hit by a foreign air force since its war with Iraq in the 1980s.
The White House was warned shortly before the strikes began but the US was not involved, according to reports.
Israel in recent weeks had been planning a response to a ballistic-missile attack by Iran on October 1, Tehran's second direct attack on Israel in six months.
The first came on April 1 - with the IDF saying "99 per cent" of Iran's missiles were shot down or destroyed.
Tehran launched an unprecedented blitz in October - aiming at least 180 ballistic missiles into Israeli territory.
It caused only minimal damage and a few injuries.
Iran said the barrage was in retaliation for attacks in recent months that killed leaders of Hezbollah, Hamas and the Iranian military - including Hassan Nasrallah, Hezbollah leader.
Israeli Prime Minister Benjamin Netanyahu immediately said Iran had made a big mistake.
Before the October attack, Israel had landed a series of devastating blows against Hezbollah, which has been firing rockets into Israel near-daily for over a year ever since the deadly Hamas attack against Israel that sparked the war in Gaza.
A few days ago secret plans showing Israel's preparation for a potential revenge strike on Iran were leaked, US officials said.
The alleged documents, marked as "top secret", were said to contain US intelligence analysing Israel's inevitable retaliation plan against Iran.
They reportedly contained satellite imagery appearing to show Israel's preparation of munitions and military drills, the reports.
Other reports said the Israeli Air Force was prepping long-range ballistic and air-to-surface missiles - believed to be for a potential strike against Iran.
The documents also suggest Israel was conducting covert surveillance on Iran using advanced drone technology.
The Sun is not directly quoting or showing the information from the leaked documents.
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Israel launched a ground operation into Lebanon earlier this month to attack Hezbollah and push the terror group away from the border to allow displaced Israelis to return.
This has stoked existing tensions in the region with more than 2,500 people in Lebanon killed in the five weeks since Israel invaded.
Israeli authorities have said 59 people have been killed in northern Israel and the occupied Golan Heights.
The Israeli military has issued evacuation warnings for civilians in Beirut - where many fierce strikes have been concentrated.
IDF Col. Avichay Adraee said during one such warning: "You are in the vicinity of Hezbollah facilities, which the IDF will operate against in the near future."
Israel is now at war with Hamas in Gaza and Hezbollah in Lebanon - both Iran-backed militant groups.
The IDF have been striking parts of northern Gaza, claiming they are aiming to wipe out Hamas brutes.
But the strikes have also killed scores of Palestinians who have been unable to flee the densely populated area.
More than 42,000 Palestinians have been killed in the war so far, according to Gaza-run health centres.
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The figure also includes Hamas terrorists, but Israel has not specified the exact ratio of civilian to militant.
Hamas killed around 1,200 Israelis and foreign nationals in the October 7 attack last year, and kidnapped around 250 more into Gaza.