CAIRO/DUBAI (Reuters) – Hamas leader Ismail Haniyeh was assassinated in the Iranian capital Tehran early on Wednesday morning, an attack that drew threats of revenge on Israel and fueled further concern that the conflict in Gaza was turning into a wider Middle East war.

The Palestinian Islamist militant group and Iran’s Revolutionary Guards confirmed Haniyeh’s death. The Guards said it took place hours after he attended a swearing-in ceremony for Iran’s new president.

Although the strike on Haniyeh was widely assumed to have been carried out by Israel, Prime Minister Benjamin Netanyahu’s government made no claim of responsibility and said it would make no comment on the killing.

Haniyeh was killed by a missile that hit him “directly” in a state guesthouse where he was staying, Khalil Al-Hayya, a senior Hamas official, told a news conference in Tehran, quoting witnesses who were with Haniyeh. “Now we are waiting for the full investigation from the [Iranian] brothers,” Al-Hayya said.

Haniyeh, normally based in Qatar, had been the face of Hamas’s international diplomacy as the war set off by the Hamas-led attack on Israel on October 7 has raged in Gaza. He had been taking part in internationally-brokered indirect talks on reaching a ceasefire in the Palestinian enclave.

The assassination occurred less than 24 hours after Israel claimed to have killed Hezbollah’s most senior military commander in the Lebanese capital Beirut in retaliation for a deadly rocket strike in the Israeli-occupied Golan Heights.

Two Lebanese security sources said on Wednesday that the body of Hezbollah operations chief Fuad Shukr had been found in the rubble of a building hit by an Israeli airstrike in Beirut’s southern suburbs.

Mr. Netanyahu made no mention of Haniyeh’s killing in a televised statement on Wednesday evening but said Israel had delivered crushing blows to Iran’s proxies of late, including Hamas and Hezbollah, and would respond forcefully to any attack.

“Citizens of Israel, challenging days lie ahead. Since the strike in Beirut there are threats sounding from all directions. We are prepared for any scenario and we will stand united and determined against any threat. Israel will exact a heavy price for any aggression against us from any arena,” he said.

The latest events appear to set back chances of any imminent ceasefire agreement in the nearly 10-month-old war in Gaza between Israel and the Iran-backed Hamas.

Hamas’ armed wing said in a statement Haniyeh’s killing would “take the battle to new dimensions and have major repercussions.” Vowing to retaliate, Iran declared three days of national mourning and said the U.S. bore responsibility because of its support for Israel.

Israel Invites ‘Harsh Punishment’, Khamenei Says

Iran’s Supreme Leader Ayatollah Ali Khamenei said Israel had provided the grounds for “harsh punishment for itself” and it was Tehran’s duty to avenge Haniyeh’s death. Iranian forces have already made strikes directly on Israel earlier in the Gaza war.

Israeli government spokesperson David Mencer told a briefing with journalists that Israel was committed to Gaza ceasefire negotiations and securing the release of Israeli hostages held by Palestinian militants in Gaza.

U.S. Secretary of State Antony Blinken, at an event in Singapore, sidestepped a question on Haniyeh’s killing, saying a ceasefire deal in Gaza was key to avoiding wider regional escalation. He told Channel News Asia that the U.S. had neither been aware of nor involved in the killing.

Qatar, which has been brokering talks aimed at halting the fighting in Gaza along with Egypt, condemned Haniyeh’s killing as a dangerous escalation of the conflict.

“Political assassinations and continued targeting of civilians in Gaza while talks continue leads us to ask, how can mediation succeed when one party assassinates the negotiator on other side?” Prime Minister Sheikh Mohammed bin Abdulrahman Al Thani said on X.

Egypt said Haniyeh’s assassination showed a lack of political will on Israel’s part to calm tensions.

Palestinian President Mahmoud Abbas condemned the killing and Palestinian factions in the Israeli-occupied West Bank called for a strike and mass demonstrations.

In Israel, the mood was buoyant as Israelis welcomed what they saw as a major achievement in the war against Hamas. Residents in besieged Gaza feared Haniyeh’s death would prolong the fighting that has devastated the enclave.

“This news is scary. We feel that he was like a father to us,” said Gaza resident Hachem Al-Saati.

Meshaal Is Likely Successor to Haniyeh

Haniyeh’s most likely successor is Khaled Meshaal, his deputy-in-exile who lives in Qatar, analysts and Hamas officials said. Under Mr. Meshaal, Hamas emerged as an ever more important player in the Middle East conflict due to his charisma, popularity and regional standing, analysts said.

Mr. Meshaal narrowly survived an attempt on his life in Jordan ordered by Mr. Netanyahu in 1997.

Appointed to the top Hamas job in 2017, Haniyeh moved between Turkey and Qatar’s capital Doha, escaping the travel curbs of the blockaded Gaza Strip and enabling him to act as a negotiator in the truce talks or to talk to Hamas’ ally Iran. Three of his sons were killed in an Israeli airstrike in April.

His deputy Saleh Al-Arouri was killed in January by Israel, leaving Yehya Al-Sinwar, the Hamas chief in Gaza and the architect of the October 7 attack on Israel, and Zaher Jabarin, the head of the group in the West Bank, in place but in hiding.

That assault by Hamas-led fighters killed about 1,200 people in southern Israeli communities and some 250 people were taken to Gaza as hostages, Israeli tallies say.

In response, Israel launched a ground and air offensive in the coastal enclave that has killed more than 39,400 people, according to Gaza health officials, and left more than 2 million facing a humanitarian crisis.

No end appears to be in sight for Israel’s campaign there as the ceasefire talks falter.