Iran's Supreme Leader Ali Khamenei said Israel had provided the grounds for "harsh punishment for itself" following the assassination of Hamas leader Ismail Haniyeh.
Iran's Revolutionary Guards confirmed the death of Haniyeh, hours after he attended a swearing-in ceremony for the country's new president, and said it was investigating.
Haniyeh, who was normally based in Qatar, had been the face of Hamas's international diplomacy as the war set off by the Hamas-led attack on Israel on October 7 has raged in Gaza.
Now, the Supreme Leader said Israel had provided the grounds for "harsh punishment for itself" and it was Tehran's duty to avenge the Hamas leader's death.
In a statement, he said: "The criminal and terrorist Zionist regime with this action, has prepared the ground for a harsh punishment for itself and we consider it our duty to avenge the blood of him who was martyred in the land of Islamic Republic of Iran."
Iranian forces had already made strikes directly on Israel earlier in the Gaza war. There was no comment nor claim of responsibility from Israel. The Israeli military said it was assessing the situation but had not issued any new security guidelines for civilians.
The assassination, which took place 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 the 10-month-old war in Gaza.
A senior Hamas official said: "This assassination by the Israeli occupation of Brother Haniyeh is a grave escalation that aims to break the will of Hamas."
Qatar, which has been brokering talks aimed at halting the fighting in Gaza, condemned Haniyeh's killing as a dangerous escalation of the conflict. China, Russia and Turkey also condemned it.
Iran's top security body met to decide Iran's strategy in reaction to the death of Haniyeh, a close ally of Tehran, a source with knowledge of the situation said.
Palestinian President Mahmoud Abbas condemned the killing and Palestinian factions in the Israeli-occupied West Bank called for a strike and mass demonstrations.
The International Criminal Court prosecutor's office requested an arrest warrant for Haniyeh over alleged war crimes at the same time it issued a similar request against Israeli Prime Minister Benjamin Netanyahu.
Responding to reports of the assassination, US Secretary of State Anthony Blinken said nothing should take away from the importance of a sustainable ceasefire.
He said: "I think nothing takes away from the importance of, as I said a moment ago, getting to the ceasefire, which is manifestly in the interests of the hostages and bringing them home.
"It's manifestly in the interests of Palestinians who are suffering terribly every single day. Children, women, and men in Gaza who've been caught in this crossfire of Hamas’ making. It's profoundly in the interest of trying to put things on a better path, not only in Gaza, but actually throughout the region, because so much is tied to what's happening in Gaza right now."
More than 39,445 Palestinians have been killed and 91,073 have been injured in Israel's military offensive on Gaza since October 7, the Gaza health ministry said in a statement earlier today.
