Hamas's top political leader Ismail Haniyeh has been killed in Iran, the group has said.
Iran's Revolutionary Guards said Haniyeh and one of his bodyguards were "assassinated" in the capital Tehran, hours after he attended a swearing in ceremony for the country's new president, Iranian state media reported.
It said it was investigating the attack and did not say how it occurred.
Israel had vowed to kill Haniyeh and other leaders of Hamas over the group's October 7 incursion on Israel that killed 1,200 people and saw some 250 others taken hostage.
There has been no immediate comment from Israeli authorities. Israel often does not comment on assassinations carried out by its Mossad intelligence agency.
Hamas said Haniyeh, who celebrated the October 7 attack, was killed "in a Zionist airstrike on his residence in Tehran" early on Wednesday, after he attended Tuesday’s swearing-in of Iran's President Masoud Pezeshkian.
"Hamas declares to the great Palestinian people and the people of the Arab and Islamic nations and all the free people of the world, brother leader Ismail Ismail Haniyeh a martyr," Hamas said in its statement.
The news, which comes less than 24 hours after Israel claimed to have killed the Hezbollah commander it said was behind a deadly strike in the Israeli-occupied Golan Heights, appears to set back chances of any imminent ceasefire agreement in Gaza.
"This assassination by the Israeli occupation of Brother Haniyeh is a grave escalation that aims to break the will of Hamas," senior Hamas official Sami Abu Zuhri told news agency Reuters.
He said Hamas would continue the path it was following, adding: "We are confident of victory."
Palestinian President Mahmoud Abbas has called the assassination of Haniyeh "a cowardly act and a dangerous development".
Turkey has also condemned the assassination of the Hamas leader, saying the attack aimed to spread the war in Gaza on a regional level.
"It has been revealed once again that the government of (Israeli Prime Minister Benjamin) Netanyahu has no intention of achieving peace," Turkey's foreign ministry said in a statement.
Iran's foreign ministry spokesman Nasser Kanaani said Haniyeh's "blood will never be wasted".
"Haniyeh's martyrdom in Tehran will strengthen the deep and unbreakable bond between Tehran, Palestine, and the resistance," Mr Kanaani was quoted as saying by Iranian state media .
Haniyeh has been the face of the Palestinian group's international diplomacy as the war set off by the Hamas-led attack on Israel on October 7 has raged in Gaza.
He left the Gaza Strip in 2019 and had since lived in exile in Qatar. The top Hamas leader in Gaza is Yehya Sinwar, who masterminded the October 7 attack.
In April, an Israeli airstrike in Gaza killed three of Haniyeh's sons and four of his grandchildren.
In an interview with news outlet Al Jazeera at the time, he said the killings would not pressure Hamas into softening its positions amid ongoing ceasefire negotiations with Israel.
“The blood of my sons is not dearer than the blood of our people,” he said at the time.
