New Delhi, Aug 1 : His first full formal day as Iran's President couldn't have started on a worse note for Masoud Pezeshkian. Hours before daylight, came the shock news of the assassination of Hamas political bureau chief Ismail Haniyeh, a guest at his inauguration, exacerbating what is already a toxic atmosphere in the Middle East.

How this development will pan for Iran's new government and if could presage escalation to a much wider conflict in the volatile region beyond Gaza and the Israel-Lebanon border remains to be seen, but the initial effect will be on the off-and-on negotiations for a halt to fighting in Gaza and the release of the remaining hostages.

Details of the assassination incident are still sketchy but according to the Islamic Revolutionary Guard Corps, a residence for war veterans in north Tehran where the Hamas leader was living was hit by a projectile at around 2 a.m., killing him and one of his bodyguards. There have not been any claims of responsibility, but Iran and the Hamas have blamed Israel.

The development is awkward for Iran, as the incident came hours after representatives from 88 countries were in the country for Pezeshkian's inauguration and its response was unequivocal and strident.

Supreme Leader Ayatollah Seyyed Ali Khamenei said that by assassinating Haniyeh, the Israeli regime has "prepared the ground for harsh punishment for itself", as per Iranian media reports.

Iran considers its duty to take revenge for the assassination of Haniyeh who was martyred in the territory of the Islamic Republic of Iran, he added.

Pezeshkian's reaction was no less strong, as he stressed that the country would defend its "territorial integrity and dignity".

"...Yesterday I raised his victorious hand and today I have to carry his coffin on my shoulders", he said in a post on X.

"The bond between the two proud nations of Iran and Palestine will be stronger than before, and the path of resistance and defence of the oppressed will be followed stronger than ever", he said, adding that Iran "will defend its territorial integrity and honour and would make the terrorist occupiers regret their cowardly action".

The IRGC, in a statement, warned "the Zionist regime" of a "harsh and painful response" for the assassination.

Meanwhile, the National Security and Foreign Policy Committee of the Iranian Parliament was slated to hold an emergency meeting on Wednesday itself over the assassination.