Iran has vowed revenge after Israel assassinated Hamas political leader Ismail Haniyeh in Tehran early Wednesday and the head of Lebanon's Hezbollah military wing, Fouad Shukr, in Beirut within 10 hours of each other, as the twin Israeli assassinations crushed hopes of an imminent ceasefire in Gaza and raised fears of a "dangerous escalation" in the region, experts said.

While Israel did not directly announce the attack on Haniyeh, there was little doubt that Israel was behind the assassination, according to the consensus of analysts and experts.

Khalil al-Hayya, a senior Hamas official, said at a news conference in Tehran, according to US media, that Haniyeh was killed by a missile that hit him "directly" in his bedroom in a government guest house where he was staying.

Haniyeh was visiting Iran to attend the inauguration of new Iranian President Masoud Pezeshkian, who said after the killing that his country would defend its territorial integrity and honor.

For his part, Iran’s Supreme Leader Ayatollah Ali Khamenei blamed Israel and said Iran had a “duty” to retaliate because Haniyeh was targeted while he was a guest in the country. The New York Times reported that Khamenei had ordered Iran to strike Israel directly, citing three Iranian officials familiar with the matter (according to the New York Times).

Experts believe the timing and location of the twin attacks, which targeted very high-ranking leaders in densely populated capitals, made them particularly humiliating for Iran and Hezbollah, raising the risk of a slide toward a full-blown regional war as Tehran seeks to re-establish a military deterrent.

According to the New York Times, "Although Hamas has vowed revenge, after nearly 10 months of fighting in Gaza, it has little ability to inflict damage outside the Strip."

Security experts and officials in Israel, Iran and Lebanon agree that an all-out conflict would be devastating for all sides, regardless of who emerges victorious. But with so much effort being made to project power in a regional proxy war, the risk of miscalculation and fatal mistakes is growing.

UN Secretary-General Antonio Guterres said he believed the attacks represented a “dangerous escalation.” His spokesman, Stephane Dujarric, said in a statement that UN Secretary-General Antonio Guterres believes the attacks represent a “dangerous escalation.” “Restraint alone is not enough at this extremely sensitive time,” he added.

The killing of Haniyeh, who played a key role for Hamas in negotiating a ceasefire and the release of hostages in Gaza, has led many to question whether the Israeli government has any real desire to stop the conflict there.

Asked by a Jerusalem correspondent on Wednesday whether he believed negotiations were still viable after Israel assassinated Ismail Haniyeh, the chief negotiator with the United States and Israel (though not directly, via the Qataris), Foreign Ministry spokesman Vedant Patel said, “I would say again, I am not going to speculate on how it (Haniyeh’s assassination) may or may not affect the ceasefire agreement.”

“What I can say is that the United States is unequivocally focused on continuing to work to bridge the gaps. We believe that reaching an agreement is not only in the interest of the United States; it is in the interest of the region, it is in the interest of Israel, and it is in the interest of the Palestinian people. We are talking about creating the conditions that allow the remaining hostages to return home, to allow humanitarian aid to flow, and to get this region out of this endless cycle of violence. That is what we will continue to focus on,” he added.

Egypt and Qatar, which played a key role in the talks, warned that Haniyeh's killing would derail the negotiations.

“How can mediation succeed when one side assassinates the negotiator on the other side?” Qatari Prime Minister Sheikh Mohammed bin Abdulrahman Al Thani wrote on Twitter.

On Wednesday evening, Israeli Prime Minister Benjamin Netanyahu gave a defiant speech, celebrating the raid in Lebanon — for which Israel has officially claimed responsibility — and vowing to continue fighting in Gaza.

Netanyahu said: "Not a week has passed in months without people at home and abroad asking me to end the war. I did not surrender to those voices then and I will not surrender to them today."

“If we had given in to the pressure to end the war, we would not have been able to eliminate the Hamas leaders, we would not have been able to control the Philadelphi Corridor [along the Egyptian border], which is the oxygen for Hamas, and we would not have been able to create the conditions necessary to return all of our abductees and achieve the goals of the war.”

The US administration has been leading international diplomatic efforts for months to prevent the war in Gaza from spreading into a wider regional conflict, and US diplomats have recently pushed hard for a ceasefire agreement in the Strip.

US Secretary of State Antony Blinken said on Wednesday that Washington was not aware of or involved in Haniyeh's assassination, and that reaching a ceasefire agreement in Gaza remains vital.

The path to de-escalating regional conflicts with Iran and its allies, from Hezbollah on Israel's northern border to the Houthis in Yemen, runs through Gaza, Blinken added.

"We do not believe that escalation is inevitable ... and there are no signs that escalation is imminent," White House National Security Spokesman John Kirby said.

At an emergency meeting of the UN Security Council, China, Russia, Algeria and other countries condemned Haniyeh's assassination, which Iran's UN ambassador called an act of terrorism. China's UN ambassador, Fu Cong, said the failure to reach a ceasefire in Gaza was responsible for the escalation of tensions.

Palestinian actress Fidaa Abdel Hadi Nasser condemned Haniyeh's killing, saying: "Violence and terrorism are Israel's main and only currency." She added: "There is no red line for Israel. No law it will not break, no standard it will not trample. No act is too corrupt or barbaric."

Haniyeh's funeral will be held in Iran on Thursday, and the country has declared three days of mourning. His body will then be flown to the Qatari capital, Doha, for burial.

Despite the shock of his death, Hamas officials and analysts said his passing would not have a major immediate impact on the ground in Gaza.

Previous Israeli assassinations of Hamas leaders

The Guardian newspaper said that the Israeli Mossad has a long history of carrying out covert operations, including many assassinations of leaders of the movement classified as "terrorist" in the United States, Israel and European countries.

The following are the most prominent of these leaders and how they were assassinated:

Yahya Ayash

On January 5, 1996, Yahya Ayyash, a leader in the Izz ad-Din al-Qassam Brigades, the military wing of Hamas, was assassinated by a mobile phone explosion in Gaza. The Israeli internal security service (Shin Bet) was accused of involvement in the killing of Ayyash, who was known as “the engineer.” He was indeed an engineer, but Israel called him “the engineer of suicide operations.”

Salah Shehada

Israel assassinated the founder of Hamas's military wing, Salah Shehadeh, in an air strike on a building in Gaza on July 22, 2002. The bombing killed 15 civilians, including Shehadeh's wife, daughter and eight other children.

Ismail Abu Shanab

Ismail Abu Shanab, one of the movement's founders and one of its most prominent political leaders, was assassinated in an Israeli missile attack on his car on August 22, 2003.

Ahmed Yassin

Ahmed Yassin was assassinated in an Israeli helicopter raid at dawn on March 22, 2004, targeting the crippled sheikh shortly after he left a mosque in Gaza.

Abdul Aziz Al-Rantisi

Less than a month later, Sheikh Yassin's successor as leader of the movement, Abdel Aziz al-Rantisi, met the same fate in an Israeli strike.

Sheikh Khalil

In September of the same year, the movement's official, Izz al-Din Sheikh Khalil, was killed in a car bomb explosion.

Nizar Ryan

Nizar Rayyan, one of the movement's most prominent political and military leaders, was killed on January 1, 2009, in a raid during an Israeli military operation that also claimed the lives of his wives and ten of his twelve children.

Mahmoud Al-Mabhouh

Mahmoud al-Mabhouh, a senior Hamas military official, was found dead in a Dubai hotel room on January 20, 2010. The movement and the emirate's police accused Israeli agents of being behind the operation, using forged foreign passports and disguising themselves as tourists or tennis players arriving from various European airports.

Ahmed Al-Jabri

Israel launched Operation Pillar of Cloud against the Palestinian resistance factions in the Gaza Strip by assassinating the deputy commander of the Qassam Brigades, Ahmed al-Jaabari, on November 14, 2012, through a missile attack that targeted his car.

Saleh Al-Arouri

Months after the outbreak of the October 2023 war in the Gaza Strip, the deputy head of the movement's political bureau, Saleh al-Arouri, and a number of his comrades were assassinated in an airstrike attributed to Israel, which targeted a building in the southern suburbs of Beirut on January 2, 2024.