Iran's Supreme Leader Ayatollah Ali Khamenei has condemned the assassination of Hamas political bureau leader Ismail Haniyeh, saying Iran considers it a duty to "avenge the dear guest's blood."

The statement came hours after Haniyeh, who was in Tehran to attend Iranian President Masoud Pezeshkian's swearing-in ceremony, was killed at his residence in the capital city in the early hours of Wednesday morning.

Khamenei stressed that the "criminal and terrorist Zionist regime" has prepared the ground for "severe punishment for itself," adding that Iran considers it a "duty" to "avenge the blood" of the Hamas leader.

He hailed Haniyeh as "a brave leader and distinguished Palestinian fighter," adding that the resistance front is "in mourning."

"The criminal and terrorist Zionist regime martyred our dear guest in our house and made us mourn, but it has also prepared the ground for severe punishment for itself," the statement read.

Haniyeh along with Islamic Jihad leader Ziad Nakhaleh had met with Iran's supreme leader on the sidelines of Pezeshkian's swearing-in ceremony on Tuesday.

The statement noted that Haniyeh was always "ready for martyrdom, and had sacrificed his children and relatives on this path," referring to the killing of his immediate family in recent months in Gaza.

"We consider it our duty to avenge his blood in this bitter and difficult incident that took place in the territory of the Islamic Republic," the statement added.

Earlier, sections of Iranian media reported that Haniyeh was assassinated in an airstrike that targeted his residence in north Tehran around 2 a.m. local time (1030GMT).

The attack reportedly took place at a guesthouse for war veterans managed by the Islamic Revolutionary Guard Corps (IRGC).

Meanwhile, the IRGC confirmed the assassination of Haniyeh and one of his bodyguards in a statement. The Guards announced that an investigation into the incident is currently underway.

Hamas also issued a statement in which it vowed to take revenge for the assassination of its leader, whom it said was killed in Tehran by a "treacherous Zionist raid."

Earlier, the Israeli army declined to comment on Haniyeh's assassination, saying, "We do not comment on these reports" in response to an Anadolu correspondent's inquiry.

Haniyeh was re-elected as the head of the Hamas political bureau in 2021 for a second consecutive term, which was slated to end in 2025.