The car crashed into a checkpoint on the US side of the Rainbow Bridge in Niagara Falls at high speed and caught on fire, according to Sky's US partner network NBC News.

Senior law enforcement officials said there was no indication that a bomb was inside, as an initial search did not find a secondary explosive or device.

New York Governor Kathy Hochul said there was "no indication of a terrorist attack".

Authorities are trying to determine whether the crash was intentional, and the FBI has warned that the situation is "very fluid".

An initial assessment suggests the incident was caused by a reckless driver.

The Rainbow Bridge, which connects the two nations across the Niagara River, has been shut along with three other border crossings in the area.

Meanwhile, the Buffalo-Niagara International Airport in New York state began security checks on all cars and told passengers to expect additional screenings.

The incident comes a day before Thanksgiving - a US public holiday when millions of Americans visit friends and family. Many people will have already started their journeys.

Canada's public safety minister Dominic LeBlanc said it was too soon to say if the crash was a deliberate act.

'Ball of fire'

Images and video purportedly from the border control scene showed a blaze and debris on the ground. A security booth was singed by flames.

Witness Mike Guenther said he saw a vehicle speeding towards the crossing from the US side of the border when it swerved to avoid another car, crashed into a fence and exploded.

"All of a sudden he went up in the air and then it was a ball of fire like 30ft or 40ft high," he told WGRZ-TV.

Ivan Vitalii, a Ukrainian man visiting the area, told The Niagara Gazette that he and a friend were near the bridge when they "heard something smash".

"We saw fire and big, black smoke," he told the newspaper.

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Speaking in Canada's parliament, Prime Minister Justin Trudeau said: "This is obviously a very serious situation in Niagara Falls.

"We are taking this extraordinary seriously."

He said "additional measures" were being contemplated and activated at border crossings across Canada.