A gunman shot dead six worshippers at a Sikh temple before opening fire on officers in what police described as an act of U.S. "domestic terrorism," The Guardian said.

The attack began shortly before 10.30am local time on Sunday as dozens of people were gathering at a place of worship in Oak Creek, Wisconsin.

After killing a number of people inside the temple, the gunman then fired on a police officer who was helping a wounded victim outside the building. A second officer fired on the suspect, who died at the scene. The wounded officer – who was shot multiple times – was taken to hospital for surgery. He is expected to survive.

It is thought that around 50 people were in the temple as the shootings took place. Prominent members of the local Sikh community were among those thought to have been injured or killed by the gunman.

CNN reported that two handguns were recovered by forensic experts at the scene. But initial fears that the gunman may have had an accomplice appear to be unfounded. There are often reports of multiple gunmen at incidents of mass shootings, because of the confusion, large number of witnesses and different perspectives. Police said sweeps of the temple and the surrounding area suggested that the shooting was the act of a lone killer.

The incident in Wisconsin comes just 16 days after 12 people were killed and nearly 60 injured in a mass shooting at a movie theatre in Aurora, Colorado, during a midnight screening of The Dark Knight Rises.