Hamas says its political leader Ismail Haniyeh has been killed by an Israeli attack in Iran. The Islamic group has been in a fierce battle with the Israeli military in the Gaza Strip.

Haniyeh was in Tehran to attend the swearing-in ceremony for Iran's new president, Masoud Pezeshkian. The ceremony was also attended by senior representatives from the Lebanon-based Shia Muslim group Hezbollah and Houthi rebels in Yemen.

Iran's state newspaper reported that Haniyeh was killed in an airstrike at about 2 a.m. on Wednesday while staying in northern Tehran.

Lebanese media has quoted Iranian sources as saying Haniyeh was killed in a missile strike.

Hamas' armed wing said in a statement that Haniyeh's killing would take the battle to "new dimensions" and have "major repercussions."

Iran's supreme leader Ayatollah Ali Khamenei condemned Israel, saying it had "prepared the ground for a severe punishment." He also said it was Iran's duty to "take revenge" for Haniyeh's death.

The Israeli government has not commented on the attack that killed Haniyeh. The Israeli military said it was assessing the situation but had not issued any new security guidelines.

The Israeli military said on Tuesday that it conducted an airstrike on the Lebanese capital of Beirut targeting a senior Hezbollah commander. It has been stepping up its offensive on forces that are showing solidarity with Hamas. The latest killing of the Hamas political leader is feared to escalate tensions in the region.