Pakistan officials say an explosion has hit a Shiite Muslim religious procession, killing 13 people.

Authorities say at least 20 people were wounded Sunday in the blast in the Rahim Yar Khan district, in Punjab province.

Police say it is not immediately clear what caused the explosion. They say they are investigating whether the blast was caused by a bomb or a transformer.

The explosion came as Shiites were marking the 40th day of mourning of the death of the Prophet Mohammed's grandson, Imam Hussain.