Iraq's foreign ministry in a statement described the killing as a "flagrant violation of international law and a threat to security and stability in the region".

The powerful Kataeb Hezbollah, part of a pro-Iran alliance of armed groups, said by killing Haniyeh, Israel has "violated all rules of engagement".

The Al-Nujaba movement, from the same alliance, earlier said "the Zionists and Americans have opened the doors of hell" following the killing of Haniyeh, the targeting of a Hezbollah commander in Beirut and a US strike on pro-Iran combatants in Iraq.

These "assassinations... will not deter us, but strengthen our determination", it added.

In recent months, the Islamic Resistance in Iraq, a loose alliance of Iran-backed groups including Kataeb Hezbollah and Al-Nujaba, has claimed to have targeted Israel with drone and rocket strikes against targets.

The Israeli army, without naming an attacker, has confirmed several aerial attacks from the east since April, but has said they were all intercepted before entering its airspace.

The alliance had also claimed attacks on US troops in Syria and Iraq for more than three months, as regional tensions soared over the Israel-Hamas war in Gaza, before suspending them in late January.

Hamas said Wednesday that Haniyeh was killed in an Israeli strike in Iran, where he was attending the swearing-in of the newly-elected President Masoud Pezeshkian, and vowed the act "will not go unanswered".

The strike came after Israel hit a Hezbollah stronghold in south Beirut on Tuesday, targeting a senior commander of the Lebanese militant group -- his fate remains unknown.

Last night, US forces carried out a "defensive" air strike on pro-Iran Iraqi fighters who were attempting to launch drones that were deemed a threat to American and allied troops, a US official said.

The strike -- which Iraqi sources said left at least four people dead -- was the first by American forces in Iraq since February.