Iran vows vengeance after assassination of Hezbollah leader Hassan Nasrallah

Iran’s supreme leader has warned Israel that its assassination of Hezbollah’s veteran leader, Sayyed Hassan Nasrallah, will “not go unavenged”, as fears of a spiralling conflict in the Middle East grow.

As the shockwaves from Friday evening’s airstrike that killed Nasrallah reverberated through the region, and Israel continued to pound targets in Lebanon, Ayatollah Ali Khamenei announced five days of official mourning on Saturday and called for an urgent meeting of the 57-member Organisation of Islamic Cooperation.

Israel’s security forces were also put on high alert for possible retaliation, as experts warned the region was facing several scenarios following the killing, including the risk of a widening war that could draw Iran, Hezbollah’s main backer, into a direct conflict with Israel.

Israeli Prime Minister Benjamin Netanyahu said on Saturday that the killing of Nasrallah was a historical turning point that could change the balance of power in the Middle East though he warned of “challenging days” ahead.

“Nasrallah was not a terrorist, he was the terrorist,” Netanyahu said in a statement.

“Nasrallah’s killing was a necessary step toward achieving the goals we have set, returning residents of the north safely to their homes and changing the balance of power in the region for years to come.”

US president Joe Biden called Nasrallah’s assassination “a measure of justice for his many victims, including thousands of Americans, Israelis and Lebanese civilians” as he ordered the Pentagon to enhance America’s defence posture in the region. Biden added that the US “fully supports Israel’s right to defend itself against Hezbollah, Hamas, the Houthis, and any other Iranian-supported terrorist groups”.

He added, however: “In Gaza, we have been pursuing a deal backed by the UN security council for a ceasefire and the release of hostages. In Lebanon, we have been negotiating a deal that would return people safely to their homes in Israel and southern Lebanon. It is time for these deals to close, for the threats to Israel to be removed, and for the broader Middle East region to gain greater stability.”

But other world leaders expressed fears over Nasrallah’s assassination. The UN secretary general, António Guterres, said he was “gravely concerned” over the “dramatic escalation” in Lebanon, joining others in warning of the danger of destabilisation.

Nasrallah, who led Hezbollah for more than three decades, was killed by Israel in a series of strikes on the group’s underground headquarters in Dahieh, a southern suburb of Beirut.

The Israel Defense Forces (IDF) announced his death on Saturday morning and Hezbollah confirmed the news later in the day, saying Nasrallah “had joined his fellow martyrs”. It added that the group would “continue the holy war against the enemy and in support of Palestine”. The statement, however, did not mention who would succeed Nasrallah, or how the group would respond to the assassination.

Israel continued airstrikes on Lebanon on Saturday, saying it had hit a Hezbollah target in Beirut’s southern suburbs. Heavy bombing has killed more than 700 people and displaced about 118,000 since it began. In response. Hezbollah said it had fired rockets at Israel.

The Israeli military also said it had intercepted a missile fired from Yemen at Tel Aviv, in what appeared to be an attack by Houthi rebels.

The killing of Nasrallah marks a major moment in the crisis in the Middle East and threatens to reshape the course of events in the region where he was a significant actor.

Iran considers Hezbollah to be one of its most significant assets. Khamenei, who Reuters reported had been transferred to a secure location inside Iran, had earlier called on Muslims “to stand by the people of Lebanon and the proud Hezbollah with whatever means they have”.

The US defence secretary, Lloyd Austin, told his Israeli counterpart, Yoav Gallant, that the US was determined to prevent Iran or Iran-backed groups from expanding the conflict and was committed to the defence of Israel, the Pentagon said. Russia condemned the killing of Nasrallah as “political murder” and called on Israel to stop hostilities in Lebanon.

According to sources in Israel, the Israeli security cabinet had previously pulled back from plans to kill Nasrallah, but having established he was due to attend a meeting at the command complex, approved a plan to kill him in an operation reportedly codenamed New Order. Reports in the Israeli media said the assassination had been carried out by a squadron of F-15I jets equipped with bunker-busting bombs although Israel refused to comment on claims they were US-supplied type 84 munitions.

Related: To understand the escalation in Lebanon, we must confront what Israelis are thinking | John Jenkins

The news was initially broken by military spokesperson Lt Col Nadav Shoshani in a brief post on X saying: “Hassan Nasrallah is dead.” In a statement issued shortly afterwards, the IDF said Nasrallah had been killed along with Hezbollah’s southern front commander, Ali Karki, and other commanders attending the meeting.

“Following precise intelligence from the IDF and the Israeli security establishment, IAF [Israeli air force] fighter jets conducted a targeted strike on the central headquarters of the Hezbollah terrorist organisation which was located underground, embedded under a residential building in the Dahieh area of Beirut,” the statement said.

“The strike was conducted while Hezbollah’s senior chain of command was operating from the headquarters and advancing terrorist activities against citizens of the state of Israel.”

The Israeli military said it was on “high alert” and prepared for a wider escalation.

Shoshani later predicted that Hezbollah would continue to target Israel. “We’ve seen Hezbollah carry out attacks against us for a year. It’s safe to assume that they are going to continue carrying out their attacks against us, or try to,” he said.

After Israel’s announcement, the streets of Beirut emptied. Gunfire could be heard across the city as mourners fired in the air to commemorate his death. Storefronts in Gemayzeh – an upmarket neighbourhood in east Beirut – were mostly closed.

The Lebanese army deployed throughout Beirut on Saturday afternoon, standing guard at major intersections throughout the city, probably in anticipation of public reaction. Lines formed at supermarkets as people rushed to buy basic goods, while people waited in queues at cash machines. Low-flying Israeli patrol drones buzzed overhead throughout the day.

Lebanon’s cabinet announced it would convene for an extraordinary session at 7.30pm.

In Dahieh, one resident told the Guardian: “I’m in despair, I don’t know what to feel.” Other supporters of Hezbollah struck a different tone. “The group will go on, it’s not just centred around one leader,” Fatimah, a resident of Dahieh, said from her car in downtown Beirut where she has been sleeping with her husband and son since the strikes on Dahieh started last week.

Hassan Nasrallah: the man who has led Hezbollah to the brink of war with IsraelThe Hezbollah’s second in command, Hashem Safieddine, who could succeed Nasrallah, was reportedly also targeted in Friday’s airstrike.

The IDF’s chief of staff, Herzi Halevi, said on Saturday that the elimination of Nasrallah was “not the end of our toolbox”, indicating that more strikes were planned.

The military said it was mobilising additional reserve soldiers as tensions escalated with Lebanon, activating three battalions of reserve soldiers to serve across Israel. It sent two brigades to northern Israel earlier in the week to train for a possible ground invasion.

Related: Hassan Nasrallah: the man who has led Hezbollah to the brink of war with Israel

Among Hezbollah’s constituents, Nasrallah was viewed with a prophet-like fervour, seen as the liberator of south Lebanon from Israel’s 18-year occupation. At Hezbollah rallies, supporters chant “Labaik ya Hussein and Labaik ya Nasrallah” – “O Hussein, O Nasrallah, I am here for you” – shouting their devotion to Hussein, a key figure in Shia Islam, and Nasrallah.

When Nasrallah spoke in televised addresses, supporters would tune in for guidance on political, spiritual and cultural issues. Many Lebanese people attribute the failure of the 2019 revolution to a speech by Nasrallah, when he told his supporters it was time to get off the streets, depriving the protest movement of its non-sectarian character.

Whoever replaces the enigmatic former secretary general will have to deal with an organisation that over the last year has lost almost every senior military leader and is on the back foot from the Israeli bombing campaign across Lebanon.

The death of Nasrallah also throws the fate of the Lebanese state into question. Hezbollah is deeply embedded in the state, controlling a key share of parliament and exercising influence over several ministries, such as the directorate of general security. Lebanon’s foreign policy is largely dictated by the group, particularly when it comes to neighbouring states such as Israel.