TEHERAN - Iran vowed on Monday to take "decisive" action against the "aggressor" behind the assassination of Hamas leader Ismail Haniyeh in Teheran, accusing Israel of carrying out a "cowardly" attack.

Foreign Ministry spokesman Nasser Kanaani said the killing of Haniyeh, who was in Iran for the swearing-in of President Masoud Pezeshkian, was a "flagrant violation" of international law.

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Iran will take its serious and deterrent action with "strength and decisiveness" to punish the "aggressor" based on its inherent right and the international law principles, Kanaani said at a weekly press conference.

He added that Teheran would pursue the matter through the United Nations and the Organization of Islamic Cooperation, which will hold an emergency meeting of foreign ministers on Wednesday.

Iran's Islamic Revolution Guards Corps (IRGC) has blamed Israel for the attack, vowing a "harsh and painful response." IRGC chief Hossein Salami said on Monday that Israel would "understand its miscalculation" after Teheran retaliates.

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The assassination has significantly heightened tensions in the region. While Iran has insisted it does not seek escalation, it has made clear it will not tolerate attacks on its territory or its citizens.