WASHINGTON (REUTERS) - Five people were killed and more than 40 injured when a sport utility vehicle (SUV) sped through a Christmas parade in the small town of Waukesha, Wisconsin, on Sunday (Nov 21), ploughing into dozens of people including young children.

Waukesha police chief Dan Thompson told reporters that one person was in custody and a vehicle had been recovered after the incident in the town of around 72,000 people, 32km west of Milwaukee.

Police said the number of dead and injured could change.

Mr Thompson said it was not clear whether the incident was terrorist in nature, but that there was no further danger and police had lifted an order urging people to take shelter.

A video posted online showed a red SUV driving at speed alongside and into the parade, appearing to run over more than a dozen people before crowds ran from sidewalks to offer assistance.

Mr Thompson said an officer fired shots at the vehicle in an attempt to stop the carnage. Another video on social media appeared to show police firing on the vehicle as it crashed through street barriers. Police did not believe shots had been fired from the vehicle, Mr Thompson added, contrary to earlier reports.

Nurse Jodi Singsime, 42, took shelter in a store with members of her family before going out to help the injured.

"I heard and saw the people being hit, but what you could do more than seeing is hearing, and just that sound was insane," she said.

She choked up as she described coming across a small boy on the road: "I felt his neck for a pulse and he had one but his eyes were barely open and his face - all I can remember is his sweet little innocent face - was purple. He wasn't really with us."

Ms Belen Santamaria, her husband, and their three-year-old daughter were planning to join the parade with their Catholic church. But Ms Santamaria, a 39-year-old Mexican factory worker, woke up on Sunday with a backache, so the family watched the procession from the sidewalk instead.

"The SUV came by at full speed," said Ms Santamaria. "Then I started to hear people screaming."

She hid in a restaurant with their daughter while her husband, 39-year-old delivery worker Jesus Ochoa, ran ahead to try to help injured people. He said he heard through other members of their church that roughly 10 members of the congregation, mostly Latinos and both adults and children, were injured.

"I was going to be there, walking," Ms Santamaria said. "The SUV would have hit us too."

A woman told Milwaukee's Fox6 TV station that the SUV hit a dance team of girls aged between nine 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 pom-poms 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 its 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 at about 64kmh 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," Mr 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.

US 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."

Photos and videos of Waukesha circulating on Twitter showed police cars and ambulances crowding a street decked out with Christmas lights in the aftermath of the incident, which occurred shortly after 4.30pm local time. People bundled up against the cold huddled on sidewalks as dusk fell.

The Children's Wisconsin hospital said on Twitter that it had received 15 patients as at 8pm, with no reported fatalities at that time.

Schools will remain closed on Monday for grades 4K-12, and additional counsellors will be available for students who may need support services, the district superintendent of schools said.

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 the Islamic State in Iraq and Syria terror group.