A gunman has opened fire at a Sikh temple in the midwestern U.S. state of Wisconsin.

Police say the shooting occurred Sunday morning at around 10 a.m. local time at the Sikh Temple of Wisconsin, located outside of Milwaukee.

The number of casualties is unknown, but several people have been reportedly shot.

A police operation is currently underway to detain the gunman who is believed to be holed up inside the building with hostages.