Hamas leader Ismail Haniyeh was assassinated in Tehran, Iran, on Wednesday morning, an attack that has prompted a series of threats of revenge against Israel and raised fears that the conflict in Gaza could spill over into a wider war across the Middle East.

The Palestinian militant group Hamas and the Iranian Revolutionary Guards confirmed Haniyeh's death. The Guards said the incident occurred just hours after Haniyeh attended the swearing-in ceremony of Iran's new president.

Although the airstrike on Haniyeh is believed to have been carried out by Israel, the government under Israeli Prime Minister Benjamin Netanyahu has not claimed responsibility and said it would not comment on the killing.

Khalil Al-Hayya, a senior Hamas official, told a news conference in Tehran that Haniyeh was killed when a missile "directly" hit a guesthouse where he was staying.

Haniyeh, who is often based in Qatar, has become Hamas's international diplomatic face since the war in Gaza erupted after Hamas staged an attack on October 7. He has attended several international talks aimed at reaching a ceasefire agreement for the Palestinian interior.

Netanyahu did not mention Haniyeh’s killing in a televised address on Wednesday night, but said Israel had struck a deadly blow against several Iranian proxies in recent weeks, including Hamas and Hezbollah, and would respond harshly to any attack.

"We are prepared for all situations and will stand united in the face of any threat. Israel will make aggressors pay a heavy price."

Recent developments are seen as a major setback in efforts to reach a ceasefire agreement for the nearly 10-month war in Gaza between Israel and Hamas.

In a statement, Hamas' armed wing said Haniyeh's killing would "take the war to a new level and lead to serious consequences." Iran, vowing to retaliate, declared three days of national mourning and said the US was also responsible for supporting Israel.

In Türkiye, thousands of pro-Palestinian protesters marched through the streets of Istanbul late Wednesday to protest the killing of Haniyeh.

Protesters in Fatih district held up posters with Haniyeh's photo, chanted "Murderous Israel, out of Palestine" and waved Turkish and Palestinian flags.

Washington has expressed concern about the possibility of escalation. However, White House national security spokesman John Kirby said the US believes this possibility is completely avoidable and will not happen in the near future and is continuing to work to prevent it.

The risks, he told reporters, “are somewhat heightened, and these risks make our mission of de-escalating, preventing escalation and persuading the parties to exercise restraint quite complicated.”

Photo: REUTERS/Documentary photo.

Haniyeh's assassination came less than 24 hours after Israel announced it had killed Hezbollah's top military commander in response to a deadly attack in the Golan.

Hezbollah confirmed that military commander Fuad Shukr was killed in an Israeli airstrike on the outskirts of Beirut.

Seven people were killed and 78 wounded in an airstrike in Beirut, the Lebanese Tele news agency reported on Wednesday, citing local civil defense authorities. Iranian state media said an Iranian military adviser was also killed in the attack.

Khamenei: Israel is provoking "harsh punishment"

Iran's Supreme Leader Ayatollah Ali Khamenei said Israel had laid the groundwork for "harsh punishment against them" and that Tehran had a duty to avenge Haniyeh's killing. Iranian forces have carried out several direct attacks on Israel in the lead-up to and during the Gaza war.

Analysts and Hamas say a potential successor to Haniyeh could be Khaled Meshaal, a deputy of his who lives in Qatar.

Israeli government spokesman David Mencer told reporters that Israel remains committed to pursuing a ceasefire in Gaza and releasing hostages held by militant groups in Gaza.

US Secretary of State Antony Blinken, in Singapore, dodged a question about Haniyeh’s killing, saying the ceasefire in Gaza was key to avoiding an escalation in the conflict in the region. He told Channel News Asia that the US had no information or involvement in the assassination.

Qatar, which has been acting as a mediator in talks towards a ceasefire in Gaza along with Egypt, condemned Haniyeh's killing.

Prime Minister Sheikh Mohammed bin Abdulrahman Al Thani wrote on X: "How can negotiations succeed if one side assassinates the negotiator of the other side?"

In Washington, Kirby said the ceasefire process had “not collapsed completely”, adding: “We believe the agreement that is being negotiated is worth pursuing”.

Although Israelis are celebrating, people living in Gaza are worried that Haniyeh's death will prolong the war.

"This news is really scary. We considered him as our father," said Hachem Al-Saati, a local resident in Gaza.

Nguyen Quang Minh (According to Reuters)