At least 13 people were killed Wednesday by armed "terrorists" who attacked a Shiite Muslim shrine in the southern Iranian city of Shiraz, state television said.

Forty others were wounded when "terrorists attacked the Shah Cheragh mausoleum in Shiraz", the state broadcaster said, updating an earlier toll from the judiciary's news website of nine dead.

At least one woman and two children were among those killed, Fars news agency said.

Iran's presidency in a statement promised a "severe response" and condemned "the enemies of Iran" who attempt to "divide the united ranks of the nation... through violence and terror".

Local media reported three assailants carried out the attack and that two had been detained, with Fars news agency saying "the terrorists are not Iranian".

But a local official said there was only one attacker.

"Only one terrorist was involved in this attack," judiciary chief Kazem Mousavi told state television.

The state broadcaster also reported one arrest and said that the assailant was "a terrorist affiliated with takfiri groups".

The term takfiri in Iran and in several other countries refers to radical Sunni Islamist groups.

The Shah Cheragh mausoleum is home to the tomb of Ahmad, brother of Imam Reza, the eighth Shiite imam, and is considered the holiest site in southern Iran.

Iran in June hanged a Sunni extremist who was sentenced to death for killing two Shiite clerics and wounding another in early April, the judiciary said at the time.

A 21-year-old national of Uzbek origin carried out the April 5 stabbing attack at the Imam Reza shrine, which honours one of the most revered figures in Shiite Islam.

The killings happened during the Muslim holy month of Ramadan when large crowds of worshippers had gathered at the shrine, in Iran's second-largest city Mashhad.

The assailant stabbed one of the victims 20 times, Tasnim news agency reported earlier.

That attack came days after two Sunni clerics were shot dead outside a seminary in the northern Iranian town of Gonbad-e Kavus.

The three suspects in that case, also Sunnis, were arrested in late April but were said to have "no connection with terrorist groups", state media reported at the time.

Wednesday's attack comes as Iran has been rocked by street protests since the death of Mahsa Amini last month after she was arrested by the morality police for allegedly violating the Islamic republic's strict dress code for women.

Dozens of people, mainly protesters but also members of the security forces, have been killed during the unrest. Hundreds more, including women, have been arrested.