A speeding SUV has ploughed through a Christmas parade in the US state of Wisconsin, killing at least one person and injuring more than 20, including a group of young dancers waving pompoms.

Waukesha Police Chief Dan Thompson said a person of interest was in custody and a vehicle had been recovered after the incident in the town, about 32 km west of Milwaukee.

"The vehicle struck more than 20 individuals. Some of the individuals were children and there are some fatalities as a result of this incident," Thompson told reporters.

Asked about the fatalities, Thompson said: "I don't have an exact number at this time."

Authorities said they had taken 11 adults and 12 children to six area hospitals, with more transported to hospitals by family and friends.

It was not known whether the incident was related to terrorism, but there was no further danger and an earlier shelter-in-place order in the town of around 72,000 had been lifted, Thompson added.

A video posted online showed a red sport utility vehicle ploughing through the parade, appearing to run over more than a dozen people before crowds ran from footpaths to offer assistance.

Thompson said an officer fired shots at the vehicle and no bystanders were injured. Police did not believe shots had been fired from the vehicle, contrary to earlier reports, he added. Video on social media appeared to show police firing on the vehicle as it crashed through street barriers.

A woman told Milwaukee's Fox6 TV station that the SUV hit a dance team of girls between 9 and 15 years old. She said the immediate reaction was silence, followed by screaming, running and checking on those injured, Fox6 reported. Video on social media showed small groups surrounding injured girls with white pompoms scattered around.

One witness said the driver also hit a group of "Dancing Grannies" and at least one person flipped over the hood of the SUV, according to Milwaukee's WISN-TV, an ABC affiliate.

"Members of the group and volunteers were impacted and we are waiting for word on their conditions," the Milwaukee Dancing Grannies said on their Facebook page. "Please keep the Grannies, all those injured, and all those who witnessed this horrible event in your thoughts and prayers."

Another witness estimated the SUV was going about 64 km/h when it hit the crowd parading down the street, the TV station said.

"As we were walking back in between the buildings ... we saw an SUV cross over, just put the pedal to the metal and just zooming full speed along the parade route. And then we heard a loud bang, and just deafening cries and screams from people who are struck by the vehicle," Angelito Tenorio, an alderman in nearby West Allis, told the Milwaukee Journal Sentinel.

The local CBS affiliate later showed a picture on Twitter of what appeared to be the red SUV involved with its hood crumpled and front fender hanging off, parked in a driveway.

President Joe Biden was briefed by aides about the events, a White House official said.

"The White House is closely monitoring the situation in Waukesha and our hearts go out to everyone who has been impacted by this terrible incident. We have reached out to state and local officials to offer any support and assistance as needed."

In 2015, four people were killed and 46 injured in Stillwater, Oklahoma when a woman drove into a crowd watching the homecoming parade for a football team.

A truck was deliberately driven into a Christmas market in Berlin, Germany in 2016, killing 12 people in an attack linked to Islamic State.

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