Hamas' martyred leader Ismail Haniyeh was killed in Tehran by a short-range projectile with a warhead of about 7 kg, Iran's Revolutionary Guards said in a statement on Saturday.
It added that Israel was "supported by the United States" in the attack.
Tehran's revenge for the attack will be "severe and (taken) at an appropriate time, place, and manner," said the statement, which blamed Israel - "the adventurous and terrorist Zionist regime" - for his death.
Haniyeh was killed early Wednesday in the Iranian capital where he was attending the swearing-in of the new president, Masoud Pezeshkian.
Iran and Hamas have vowed to retaliate.
The Guards repeated their insistence that Haniyeh would be avenged and that Israel would receive "a severe punishment at the appropriate time, place and manner".
Israel, which has declined to comment on Haniyeh's killing, had earlier struck a Hezbollah stronghold in south Beirut.
That strike killed a senior commander of the Lebanese resistance group it blamed for a deadly weekend rocket strike on the Israeli-annexed Golan Heights.
The killings are the latest of several major incidents that have inflamed regional tensions during the Gaza war, which has drawn in Iran-backed resistance movements in Syria, Lebanon, Iraq and Yemen.
In Iran, the voices clamouring for revenge have intensified since Haniyeh's killing.
On Saturday, the ultraconservative Kayhan daily said retaliatory operations were expected to be "more diverse, more dispersed and impossible to intercept."
"This time, areas such as Tel Aviv and Haifa and the strategic centres and especially residences of some officials involved in the recent crimes are among the targets," the newspaper said in an opinion piece.
