BEIRUT — Israeli Prime Minister Benjamin Netanyahu says assassinating Hezbollah leader Hassan Nasrallah became an “essential condition” for Israel to achieve its war goals.

In his first public remarks since the killing, Netanyahu said Saturday that the killings of other top Hezbollah commanders was not enough and he decided Nasrallah also needed to be killed.

He blamed Nasrallah for being “the architect” of a plan to “annihilate” Israel.

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Lebanon’s Hezbollah group confirmed on Saturday that its leader and one of its founding members, Hassan Nasrallah, was killed in an Israeli airstrike in a southern suburb of Beirut.

The killing of the powerful militant group’s longtime leader sent shockwaves throughout Lebanon and the Middle East, where he has been a dominant political and military figure for more than three decades.

Nasrallah has been on Israel’s kill list for decades. His assassination is by far the biggest and most consequential of Israel’s targeted killings in years. The Israeli military said it carried out a precise airstrike on Friday while Hezbollah leaders were meeting at their headquarters in Dahiyeh, south of Beirut.

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PHOTOS: Netanyahu says Nasrallah's killing was essential for Israel to achieve its war goals

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Immediately after the confirmation from Hezbollah, people starting firing in the air in Beirut and across Lebanon to mourn Nasrallah’s death.

“Wish it was our kids, not you, Sayyid!” said one woman, using an honorific title for Nasrallah, as she clutched her baby in the western city of Baabda.

“We don’t believe he is killed,” a woman draped in black, tearfully told al-Manar TV in Bekaa, western Lebanon. “We don’t. We left our homes and came here for him and for the resistance,”

The Lebanese Health Ministry said six people were killed and 91 injured in the strikes, which leveled six apartment buildings. Ali Karki, the commander of Hezbollah’s Southern Front and other commanders were also killed, the Israeli military said.

A statement from Hezbollah said Nasrallah - who led the group for more than three decades - “has joined his fellow martyrs.” The group vowed to “continue the holy war against the enemy and in support of Palestine.”

Hezbollah started firing rockets on Israel in support of Gaza on Oct. 8, a day after Hamas militants launched an unprecedented attack on Israel, killing some 1,200 people and abducting another 250. Since then, the two sides have been engaged in escalating cross-border strikes.

Earlier this month, thousands of explosives hidden in pagers and walkie-talkies used by Hezbollah detonated, killing dozens of people and maiming thousands, including many civilians. Israel is widely believed to be behind the attack. Israel has killed several other top Hezbollah commanders in Beirut, especially in the past two weeks, in addition to the attack that killed Nasrallah.

In Beirut’s southern suburbs, smoke rose and the streets were empty Saturday after the area was pummeled overnight by heavy Israeli airstrikes. Shelters set up in the city center for displaced people were overflowing. Many families slept in public squares and on beaches or in their cars. On the roads leading to the mountains above the capital, hundreds of people could be seen fleeing on foot, holding infants and whatever belongings they could carry.

The Palestinian militant group Hamas sent condolences to its ally, Hezbollah, and said “assassinations will only increase the resistance in Lebanon and Palestine in determination and resolve.”

News of Nasrallah’s killing stunned travelers at Lebanon’s only international airport, where hundreds of people were scrambling to leave the country despite limited flights. Some cried. Others talked on their phones in disbelief. One woman screamed: “No! It was just an announcement! No, he didn’t die!”

Iran’s supreme leader announced five days of public mourning after Nasrallah’s death. Ayatollah Ali Khamenei called Nasrallah “the flag-bearer of resistance” in the region.

Hundreds of protesters meanwhile took to the streets of Tehran, waving Hezbollah flags and chanting “Death to Israel” and “Death to Netanyahu the murderer.”

Thomas Juneau, a professor at the University of Ottawa’s Graduate School of Public and International Affairs, said Iran will be under significant pressure to respond to Nasrallah’s killing without escalating violence in the region.

“Iran understands that its military options are limited, given the conventional military superiority of Israel and the U.S.” Juneau told The Associated Press.

Israel’s Chief of Staff, Lt. Gen. Herzi Halevi, said Saturday that the elimination of Nasrallah was “not the end of our toolbox,” indicating that more strikes were planned. Defense Minister Yoav Gallant called it “the most important targeted strike since the founding of the State of Israel.”

Israel has vowed to step up pressure on Hezbollah until it halts its attacks that have displaced tens of thousands of Israelis from communities near the Lebanese border. The recent fighting has also displaced more than 200,000 Lebanese in the past week, according to the United Nations.

The military said Saturday it was mobilizing three more battalions of reserve soldiers to serve across the country. It already sent two brigades to northern Israel to prepare for a possible ground invasion.

Shoshani, the army spokesperson, said Israel has inflicted heavy damage on Hezbollah’s capabilities over the past week by targeting immediate threats and strategic weapons, such as larger, guided missiles. But he said much of Hezbollah’s arsenal remains intact and that Israel would continue to target the group.

Air raid sirens sounded across central Israel on Saturday afternoon, including at the Tel Aviv international airport, shortly after Prime Minister Benjamin Netanyahu returned from a trip to the U.S.

The Israeli military said it intercepted a missile launched from Yemen. It was not immediately known if the missile strike was aimed at Netanyahu’s flight.

The Israeli military updated guidelines for Israeli citizens, canceling gatherings of more than 1,000 people due to the threat.

Approximately 60,000 Israelis have been evacuated from their homes along the Lebanese border for almost a year. This month, Israel’s government said halting Hezbollah’s attacks in the country’s north to allow residents to return to their homes is an official goal.

Orna Mizrahi, a senior researcher at the Tel Aviv-based think tank Institute for National Security Studies and former intelligence analyst for the Israeli military and prime minister’s office, noted that Nasrallah was sometimes a “voice of reason,” interested in engaging Israel in a war of attrition and holding the militant group back from using the full force of their formidable arsenal against Israel.

Nasrallah’s death could prompt some less senior members of Hezbollah to unleash much stronger weapons than have been used in the nearly yearlong exchange of hostilities, Mizrahi said. The biggest question mark right now, though, is how Iran will respond.

She said Nasrallah’s death could provide a window of opportunity - while the organization is significantly weakened - for Lebanon to dilute Hezbollah’s influence, especially in the south, that threatens to drag Lebanon into a full-scale war with Israel.

On Saturday morning, the Israeli military carried out more than 140 airstrikes in southern Beirut and eastern Lebanon’s Bekaa Valley, including targeting a storage facility for anti-ship missiles in Beirut suburb of Dahiyeh. Israel said the missiles were stored beneath civilian apartment buildings. Hezbollah launched dozens of projectiles across northern and central Israel and deep into the Israel-occupied West Bank, damaging some buildings in the northern town of Safed.

The Israeli army again warned Lebanese residents to stay away from Hezbollah combat equipment and facilities, including in the southern suburbs of Beirut and southern Lebanon. The U.S. State Department issued an alert urging American citizens to leave the country.

A total of 1,030 people - including 156 women and 87 children - have been killed in Israeli strikes in Lebanon in less than two weeks, the country’s health minister said Saturday.