Top Hamas leader Ismail Haniyeh has been killed in Iran, the group has said.
In a statement released on Wednesday, Hamas said Haniyeh was killed in an Israeli attack on his residence in Tehran.
A number of other senior Hamas figures, including Palestinian President Mahmoud Abbas, have described Haniyeh's death as an "assassination" and vowed to retaliate.
The Iranian Revolutionary Guard Corps said the cause of the "incident" was not immediately clear but was "being investigated", AFP news agency reported.
According to the group, Haniyeh was killed a day after participating in the inauguration ceremony of the new Iranian President Masoud Pezeshkian who was sworn in on Tuesday.
Hamas, the Palestinian group controlling Gaza, said Haniyeh was "killed in a treacherous
Zionist raid".
The group's political bureau member Musa Abu Marzuk said it was a "cowardly act" and one which "will not go unanswered"; while another senior Hamas official, Sami Abu Zuhri, said the group will "continue on its path".
"This assassination by the Israeli occupation of Brother Haniyeh is a grave escalation that aims to break the will of Hamas and the will of our people and achieve fake goals," Sami Abu Zuhri told Reuters news agency.
Iran's foreign ministry said the "martydom" of Haniyeh will "strengthen the deep and unbreakable bond between Tehran, Palestine and the resistance," state media reported.
The Russian and Turkish foreign ministries also condemned the attack.
Israel is yet to respond or issue a statement.