Iran's Supreme Leader Ayatollah Ali Khamenei vowed on Thursday to retaliate against those threatening the country's security after a Daesh-claimed massacre of Shiite pilgrims threatens to inflame tensions amid widespread anti-government protests.

In a statement read on state TV, Ayatollah Ali Khamenei said the assailants "will surely be punished" and called on Iranians to unite.

"We all have a duty to deal with the enemy and its traitorous or ignorant agents," said Khamenei a day after the attack killed 15 people.

Khamenei's call for unity appeared to be directed at mostly government loyalists and not protesters whose nearly six-week-old movement is seen as a threat to national security by authorities.

Iran's clerical rulers have faced nationwide protests since the death in police custody of Mahsa Amini, a 22-year-old Kurdish woman, on Sept. 16.

Iranians have called for the death of Khamenei and an end to the Islamic Republic during the protests, which have become one of the boldest challenges to the clerical leadership since the 1979 revolution, drawing many Iranians onto the streets.

Authorities, in the meanwhile, said they had arrested a gunman who carried out the attack at the Shah Cheragh shrine in the city of Shiraz as state media laid blame on terrorist groups like Daesh.

A senior official said the suspected attacker was in critical condition after being shot by police.

"The shrine terrorist is in critical condition ... and we have not been able to interrogate him yet," said Deputy Provincial Gov. Easmail Mohebipour, quoted by the semi-official Tasnim news agency.

CCTV footage broadcast on state TV on Thursday showed the attacker entering the shrine after hiding an assault rifle in a bag and shooting as worshippers tried to flee and hide in corridors.

Daesh, which once posed a security threat across the Middle East, has claimed previous violence in Iran, including deadly twin attacks in 2017 that targeted parliament and the tomb of the Islamic Republic's founder, Ayatollah Ruhollah Khomeini.

Since the peak of its power, when it ruled millions of people in the Middle East and struck fear across the world with deadly bombings and shootings, Daesh has slipped back into the shadows.

Iran often accuses the West and its regional rivals Israel and Saudi Arabia of fomenting attacks. Saudi Arabia denies this and Israel usually declines to comment on its moves against Daesh.

Amini protests continue

Wednesday's killing of Shiite pilgrims came on the same day that Iranian security forces clashed with increasingly strident protesters marking 40 days since Amini's death.

Iranian human rights groups said there were unconfirmed reports that some members of Amini's family are under house arrest. Reuters could not verify these reports. Reuters tried to reach Amini's father and brother.

They also reported one more protester death in the western city of Mahabad Thursday.

"A young Kurdish man was killed by direct fire from Iranian security forces," Norway-basedHengaw Organisation for Human Rights group, said on Twitter. "This young man was shot in the forehead."

Hengaw said government forces had opened fire on people in the Gomrok neighborhood of Mahabad, in West Azerbaijan province, after one of the city's police stations was surrounded.

"We should not mourn for our youth, we should avenge them," the protesters chanted, according to the rights group.

Iranian authorities, who have accused the United States and other Western countries of fomenting what they call "riots," have yet to declare a death toll, but state media have said about 30 members of the security forces have been killed.

The activist news agency HRANA said in a posting that at least 252 protesters had been killed in the unrest, including 36 minors.

It said 30 members of the security forces were killed and more than 13,800 people had been arrested as of Wednesday in protests in 122 cities and towns and some 109 universities.