Hamas political leader Ismail Haniyeh was assassinated in Iran, the terror group said early Wednesday.
No one has claimed responsibility but Israel is assumed to have carried out the killing of the Hamas leader.
Israel had vowed to kill the leaders of Hamas after the Oct. 7 attack that killed around 1,200 people in Israel and led to about 250 hostages being taken into Gaza.
More than 100 hostages are still believed to be held.
Haniyeh was in Iran to attend the inauguration of the country's new president. He had been living in exile in Qatar.
He met with President Masoud Pezeshkian at the presidency office in Tehran hours before he was killed.
No details around his death were immediately released but Iran blamed Israel and said it will face a "harsh and painful response," the Associated Press reported.
Israel said it was not looking for an expansion of its war against Hamas into a regional conflict but was prepared for "all possibilities."
The killing of Haniyeh cam hours after Israel carried out an airstrike in southern Beirut that targeted a Hezbollah commander who allegedly ordered the rocket attack that killed 12 children and teens playing soccer in the Israeli-occupied Golan Heights.