The police spokesman said people were still trapped in the wreckage, while others were ejected on impact.
At least one child was among the dead, the spokesman said, without offering a specific death toll.
“There are many serious injuries, and as was just announced by New York State Police, there are multiple fatalities,” Erie County Executive Mark Poloncarz said on X.
At least 16 people were taken to Erie County Medical Centre, a hospital spokesperson told ABC News.
Several helicopters were involved in the rescue effort. Translators were also being brought in to help those injured.
“The driver is alive and well – we’re working with him. We believe we have a good idea of what happened, why the bus lost control. We just want to make sure that all the details are thoroughly vetted,” O’Callaghan said.
State Governor Kathy Hochul said her team was coordinating with state police and local officials “who are working to rescue and provide assistance to everyone involved”.
“I’m heartbroken for all those we’ve lost and all those injured and praying for their families. Thank you to our brave first responders on the scene,” said the senior US senator from New York, Chuck Schumer.
Niagara Falls is one of the top tourist destinations in the United States.
– Agence France-Presse