Israel has launched its heaviest air attack on Beirut in almost a year of conflict with the Lebanese militant group Hezbollah, levelling a number of buildings in a southern suburb in an apparent attempt to kill Hezbollah’s leader and a key Iran ally, Hassan Nasrallah. Six loud explosions were heard across the Lebanese capital late on Friday afternoon, and vast plumes of smoke were visible from as far as Batroun, a city an hour’s drive away. Several apartment blocks in the predominantly Shia neighbourhood of Haret Hreik were reduced to rubble, and footage from the scene showed huge slabs of concrete topped by piles of twisted metal and wreckage. Several craters were visible; a car had fallen into one. The Lebanese health ministry said six people had died and 91 were injured, while some early estimates put the number of dead at 300. More casualties are expected as rescue workers clear rubble. Video of the strikes suggested they were carried out with ground-penetrating munitions known as bunker busters. In some footage, a vertical jet of flame was visible as a bomb appeared to explode beneath the ground. Israeli media reported that Nasrallah was the principle target and that the military was checking whether he had been hit. Other media outlets quoted Hezbollah sources saying he was “alive and well”. The strikes came shortly after the Israeli prime minister, Benjamin Netanyahu, told the UN general assembly in a bellicose speech marked by the walkout of dozens of diplomats that Israel’s campaign against Hezbollah would continue despite international efforts to secure a three-week ceasefire. The Iranian president, Masoud Pezeshkian, condemned the attack as a “flagrant war crime.” “The attacks perpetrated … by the Zionist regime in the Dahiya neighbourhood of Beirut constitute a flagrant war crime that has revealed once again the nature of this regime’s state terrorism,” he said in a statement carried by the official Irna news agency. Targeting Nasrallah – even if he was not harmed – would mark a staggering escalation on the Israeli side. He represents Iran’s most important regional asset and has long been seen as linchpin in the so-called axis of resistance. The presence of Hezbollah’s large rocket arsenal on Israel’s northern border has long acted as a deterrent to an Israeli attack on Iran and its nuclear programme. The Israel Defense Forces (IDF) spokesperson, R Adm Daniel Hagari, said the strikes had hit the main Hezbollah headquarters, which he said was underground beneath residential buildings. Hagari said the IDF was still assessing the result of the attack, which he described as “very precise”, and warned that Israel would attack other Hezbollah targets in the coming hours. His comments came shortly after Israel’s Arabic-language military spokesperson Avichay Adraee posted a map showing three buildings in Dahiya in south Beirut and warned nearby residents to evacuate. Hagari also said that Israeli air force jets were patrolling around Beirut airport to prevent Iran from transferring weapons to Hezbollah. The British embassy reiterated its warning to UK citizens, posting: “British nationals in Lebanon should leave now. You should take the next available flight.” As night fell in Jerusalem, Netanyahu’s office said he had personally approved the strike, issuing a photograph of Netanyahu with his military secretary and chief of staff on the phone in his New York hotel. His office also announced that he had cut short his US visit and would return immediately to Israel. Late on Friday night, Hezbollah launched fresh rocket salvoes against the north Israeli cities of Safed, Karmiel and Sa’ar, which it said were carried out “in response to Israeli attacks on cities, villages and civilians”. Underlining the significance of the strike, Israeli media reported that the operation was watched as it unfolded by the defence minister, Yoav Gallant, in the command centre of the Israeli air forcein Tel Aviv, along with the Israeli chief of staff, Herzi Halevi, and other top commanders. Although some Israeli media suggested that the US had been informed minutes before the attack, that was emphatically denied by US president Joe Biden who told reporters the US “no knowledge of or participation” in the strike. The explosions were so powerful that they rattled windows and shook houses in settlements 18 miles north of Beirut. Nearby witnesses quoted by the Lebanese daily L’Orient-Le Jour described seeing substantial fissures open in the ground. Ambulances were seen heading to the scene of the explosions, sirens wailing. Not long before the attack, thousands of people had gathered in Dahiya for the funeral of three Hezbollah members, including a senior commander, killed in earlier strikes. Lebanon’s prime minister, Najib Mikati, who is also in New York, was following developments as information arrived, according to a statement from his office. The statement said Mikati was in touch with the commander of the Lebanese armed forces, Joseph Aoun, and had ordered “the full mobilisation” of emergency resources after reports of a large number of victims. “This new aggression demonstrates that the Israeli enemy is mocking all the international appeals in favour of a ceasefire from the international community,” Mikati said. Earlier in the day, Israeli airstrikes on Lebanon killed about 25 people, taking the death toll this week to more than 720, health authorities said. The Israeli military said it carried out dozens of strikes over the course of two hours across the south of Lebanon on Friday, including in the cities of Sidon and Nabatieh. It said it was targeting Hezbollah rocket launchers and infrastructure. It said Hezbollah fired a volley of rockets toward the northern Israeli city of Tiberias. A year of conflict between Israel and Hezbollah has escalated sharply this week, raising fears of an even more destructive conflict. More than 90,000 people have been reported as newly displaced in Lebanon this week, according to the UN, adding to more than 111,000 already uprooted by the conflict. Hezbollah began firing at Israel on 8 October last year as the Gaza war began, declaring solidarity with the Palestinians. Hezbollah has said it will cease fire only when Israel’s Gaza offensive ends. On Friday, the Moody’s credit rating agency downgraded Israel’s credit rating to “Baa1” and maintained its rating outlook at “negative” amid escalation of the conflict in the region.