Three armed gunmen attacked the shrine of Shah-e-Cheragh in Shiraz on Wednesday evening.
The assailants have reportedly fired shots at the pilgrims inside the site.
Two terrorists have been arrested, but the third one is still at large.
Initial reports said, at least 13 people have been killed and 40 others injured in the shooting attack.
Esmaeel Mohebipour, the deputy governor of Fars Province for political, security and social affairs, said the armed terrorists started a shooting rampage at the shrine at the evening prayers time.
Shah-e-Cheragh (the shrine of the Lord of Light) is a funerary monument and mosque located in the city of Shiraz, where lies the tomb of Sayyed Mir Ahmad and his brother, Sayyed Mir Mohammad, the sons of the seventh Imam and brothers of the eighth Shiite imam, Imam Reza (AS).