Iran's Supreme Leader swore to respond after the slaughter of Shi'ite pilgrims, an attack claimed by Islamic State that threatens to escalate the conflict amid anti-government protests nationwide.

Ayatollah Ali Khamenei issued a statement aired on state television in which he vowed that the attackers "will surely be punished" and urged Iranians to be united.

Khamenei added, according to Reuters, that all Iranians "have a duty " to confront the adversary and its treasonous or "ignorant agents" one day following the brutal incident at a religious site.

Islamic State claimed responsibility for a deadly attack on a Shi'ite Muslim shrine in Iran on Wednesday, in which 15 people were killed, further inflaming tensions in a country still reeling from a string of demonstrations.

A gunman responsible for the attack on the Shirazian shrine of Shah Cheragh has been reportedly apprehended, according to Iranian authorities. Iranian state media reportedly accused "takfiri terrorists," a term for radical Sunni Muslim extremists like Islamic State.

The Islamic State has claimed responsibility for past assaults in Iran, such as the 2017 dual bombings of the Iranian parliament and the mausoleum of Islamic Republic founder Ayatollah Ruhollah Khomeini, which left at least 88 people dead.

Deadly Attack Coincides With 40th Day of Widespread Protests

On Wednesday, Iranian security forces fought with increasingly strident protestors commemorating the 40th day of the death in detention of Mahsa Amini, a 22-year-old Kurdish woman. Amini's death sparked widespread outrage throughout the country.

CNN reports that Amini was arrested by "morality police" and held in a "re-education center" on September 16 for allegedly violating the country's stringent clothing code.

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According to state media, Interior Minister Ahmad Vahidi blamed Iran's demonstrations for sparking the Shiraz massacre, and President Ebrahim Raisi pledged Tehran would retaliate.

Before the Islamic State claimed responsibility for the attack, Raisi remarked that "Iran's enemies, after failing to create a split in the nation's united ranks, take revenge through violence and terror" based on experience, as per Reuters.

An earlier report by Nour news, a news organization linked with the Iranian government, stated that the culprits were not Iranian nationals.

UN Condemns Shiraz Attack

Fars News Agency reported a woman and two children died in Shiraz.

One survivor told Al Jazeera that they were "preparing for prayers" when they heard the gunshots. "We tried to escape from the other side and then I realized I was bleeding," he said.

According to Fouad Izadi, an associate professor in the Faculty of World Studies at the University of Tehran, the incident was an attempt to attack worshippers, mosques, and shrines which is a "signature of ISIS."

UN spokesperson Stéphane Dujarric said attacks on religious places are "especially heinous."

The Shiraz shrine was a popular venue for devotees making their way around the world. Two of Musa al-Kadhim, the seventh Shia Imam, and his son Ali al-Rida, the eighth Imam, are buried in the mausoleum.

The most recent assault at the shrine occurred in April 2008 when a bomb hidden in a mosque killed 14, according to BBC.

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