At least six people were killed when an off-duty deputy sheriff went on a shooting rampage early Sunday in northern Wisconsin before he was shot dead by police, authorities said.

Forest County Deputy Sheriff Tyler Peterson, 20, opened fire after entering a house party at 3 a.m. local time in Crandon, a remote city about 360 kilometres north of Milwaukee, officials from the Forest County Sheriff's Department said.

Peterson was shot and killed shortly afterward, police said.

Ten people between 17 and 20 years old were at the home at the time, witnesses said. A seventh shooting victim was in critical condition at a nearby hospital, police said.

Crandon police Chief John Dennee would not confirm details about the shooting, but a dispatcher for the State Patrol who declined to give his full name as a matter of department practice said Peterson was also a part-time officer for the Crandon police department.

Several State Patrol officers went to Forest County to help investigators, the dispatcher said.

"It's a pretty tragic situation here," said county supervisor Tom Vollmar, who lives just outside Crandon, a city of 2,000. "There are five or six people dead."

Crandon Mayor Gary Bradley declined to comment on the investigation.

"We are going to get together and be strong," he said. "We are a strong community. We always have been. This is agonizing, but we will prevail."

The area near the remote Wisconsin town is known for logging, fishing, hunting and snowmobiling.