Hamas leader Ismail Haniyeh was assassinated in Tehran, the Palestinian militant group and Iran's elite Revolutionary Guards said in separate statements on Wednesday.

Haniyeh was in Tehran to attend Iran’s President Masoud Pezeshkian’s swearing in ceremony on Tuesday. The Guards said the attack was under investigation.

"Early this morning, the residence of Ismail Haniyeh in Tehran was struck, resulting in his and one of his body guards' martyrdom. The cause is under investigation and will be announced soon," the Revolutionary Guards said.

The Islamist faction, Hamas, mourned the death of Haniyeh, who it said was killed in "a treacherous Zionist raid on his residence in Tehran".

Analysts on Iranian state television immediately began blaming Israel for the attack. No one immediately claimed responsibility for the assassination, but Israel had vowed to kill Haniyeh and other leaders of Hamas over the group’s October 7 attack on the country that killed 1,200 people and saw some 250 others taken hostage.

Israel itself did not immediately comment.

In Israel's war with Hamas since the October attack, more than 39,360 Palestinians have been killed and more than 90,900 wounded, according to health authorities in Gaza. (Agencies)