Hamas chief Ismail Haniyeh sits in a meeting with President Masoud Pezeshkian in Tehran, Iran, on Tuesday. AP

Hamas leader Ismail Haniyeh was assassinated in the early hours of the morning in Iran, the Palestinian group Hamas said on Wednesday, describing the strike as a "severe escalation" that would not achieve its goals.

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.

There was no immediate comment from Israeli authorities.

The White House did not immediately respond to a request for comment on Haniyeh.

The news, which came 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 Gaza.

Ismail Haniyeh attends Iran's new President, Masoud Pezeshkian's swearing-in ceremony in Tehran on Tuesday. Reuters

"This assassination by the Israeli occupation of Brother Haniyeh is a grave escalation that aims to break the will of Hamas," senior Hamas official Sami Abu Zuhri told Reuters.

He said Hamas would continue the path it was following, adding: "We are confident of victory."

Haniyeh, normally based in Qatar, has been the face of the Palestinian group's international diplomacy as the war set off by the Hamas-led attack on Israel on Oct. 7 has raged in Gaza, where three of his sons were killed in an Israeli airstrike.

Appointed to the Hamas top job in 2017, Haniyeh has moved between Turkey and Qatar's capital Doha, escaping the travel curbs of the blockaded Gaza Strip and enabling him to act as a negotiator in ceasefire talks or to talk to Hamas' ally Iran.

Palestinian president Mahmud Abbas attends a meeting. File photo

Palestinian president Mahmud Abbas said on Wednesday the killing of Hamas chief Ismail Haniyeh was a "cowardly act" and urged Palestinians to remain united against Israel.

"President Mahmud Abbas of the State of Palestine strongly condemned the assassination of Hamas leader Ismail Haniyeh, deeming it a cowardly act and a serious escalation," Abbas's office said in a statement. "He urged our people and their forces to unite, remain patient, and stand firm against the Israeli occupation."

Agencies