Foreign Ministry spokesman Mohammad-Ali Hosseini Sunday cautioned journalists against jumping to conclusions about the cause of the blast, Iran's official Islamic Republic News Agency, IRNA, reported.
An investigation was under way into what sparked the explosion during the Saturday evening sermon, the spokesman said.
Early reports said a bomb was in a briefcase or bag, in or near the mosque.
Shiraz is Iran's fifth-largest city, located more than 500 miles from Tehran.
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