Iran's President Ebrahim Raisi said Thursday that "riots" pave the way for "terror" attacks, a day after a deadly assault on a shrine in the southern city of Shiraz claimed by the Islamic State group.

"The intention of the enemy is to disrupt the country's progress, and then these riots pave the ground for terrorist acts," he said in televised remarks.

At least 15 people were killed Wednesday in an attack on a key Shiite Muslim shrine in Shiraz, according to official media.

State television said it was carried out by an armed "terrorist" during evening prayers at the Shah Cheragh mausoleum, and left at least 19 people wounded.

The assailant was a man "in his 30s" who had been detained by the security forces, officials said.

Raisi had vowed "a severe response" on Wednesday, according to a presidency statement.

He also condemned "the enemies of Iran" who attempt to "divide the united ranks of the nation... through violence and terror."

The shrine attack came as thousands of people paid tribute to Mahsa Amini, 40 days after her death in police custody.

Dozens of people, mainly protesters but also members of the security forces, have been killed during the unrest that was triggered by the 22-year-old's death on September 16 after her arrest by the morality police in Tehran for allegedly violating the country's strict dress code for women.

Hundreds of people, including women, have been arrested.

Iran's judiciary has pressed charges against more than 1,000 people arrested in connection to the protests.

At least four were charged with an offence that can carry the death penalty, while others accused of "acting against the country's security", "propaganda" against the regime and "assaulting security forces".

Officials had said that hundreds of those who were not involved in the "riots" had been released.