Iranian leader Sayyed Ali Khamenei has expressed condolences to the Iranian people following a deadly attack on a holy shrine in Shiraz, stressing that the perpetrators of the "heinous crime will face harsh punishment”.

In a letter, Khamenei said that the grief brought on by the loss of these pilgrims and the desecration of the shrine will not be compensated unless the perpetrators of these disastrous acts are tracked to wherever they came from, and decisive measures are taken against them.

The Iranian leader went on to say that everyone has a role to play in confronting the enemy and its "traitor or ignorant" agents.

“The dear nation and the institutions in charge will definitely overcome the sinister plot hatched by the enemies, God willing,” he added.

In a message of condolence to the martyrs' families, the people of Shiraz, and the entire Iranian nation, Khamenei prayed for the recovery of those injured.

Earlier, Iranian President Ebrahim Raisi stated that the Islamic Republic will deliver a crushing response to those responsible for the Wednesday terrorist attack on the holy shrine of Shah Cheragh in Shiraz, Iran's southwestern city.

The Iranian President affirmed that “the country’s security and law enforcement forces, having identified the root causes of this heinous crime, will deliver a decisive response to its architects and operatives," adding that this move is one that Iran's enemies committed out of desperation in order to harm Iran's united people.

The Iranian President also expressed his heartfelt condolences for the pilgrims who died during the Wednesday massacre.

ISIS claimed responsibility on Wednesday for the attack on the shrine, which left 15 killed and 40 wounded.