A vehicle speeding toward a US-Canada bridge from the American side has crashed and exploded at a checkpoint in Niagara Falls, killing two people and prompting the closing of multiple border crossings for hours.

Authorities weren't sure what spurred the wreck but said there were no signs it was a terror attack.

The FBI's Buffalo office said it had concluded its investigation: “A search of the scene revealed no explosive materials, and no terrorism nexus was identified,” it said.

“The matter has been turned over to the Niagara Falls Police Department as a traffic investigation.”

Much remained unclear about the incident at the Rainbow Bridge, which stirred concerns on both sides of the border as the US headed into the Thanksgiving holiday.

Both US President Joe Biden and Canadian Prime Minister Justin Trudeau were briefed soon afterward, and Trudeau excused himself from Question Period in the House of Commons to get further information, saying officials were “taking this extraordinarily seriously”.

A few hours later, New York Governor Kathy Hochul and western New York's US attorney Trini Ross both sought to ease fears, while cautioning that the investigation was in the early stages.

“Based on what we know at this moment,” Hochul said, “there is no sign of terrorist activity in this crash”.

Security camera video released by the US government showed the car race through an intersection on a wet road, hit a low median and vault high into the air in a US Customs and Border Protection area just east of the main vehicle checkpoint.

The car flew for yards, twisting, and then crashed into a line of booths out of the camera’s view.

Rickie Wilson, a Niagara Falls tour guide, was by his parked car nearby and turned around when he saw something in the air.

“I first thought it was an airplane. It looked like slow motion,” he said. “I said, ‘My gosh, it’s a car. It’s a vehicle, and it’s flying through the air.’”

The identities of those in the car weren't released.

Hochul said it wasn’t clear whether the driver – a western New York resident – was intentionally heading for the bridge, which crosses the Niagara River.

The two people who died were a husband and wife, according to a person briefed on the investigation.

Matthew Miraglia, the FBI special agent in charge in Buffalo, said investigators so far had found no “derogatory” information on the driver.

“We’re scanning his social media. There’s nothing there," Miraglia said.

New York Senator Chuck Schumer said he had been in contact with both the FBI and the White House and investigators had found “no connection to any terrorist or criminal group”.

Investigators swabbed the scene and found no evidence of chemicals or substances used for explosives, he added.

Officials said the car was traveling at tremendous speed as it approached the bridge at around 11.30am in downtown Niagara Falls in an area that includes several hotels and a casino.

Hochul said the car ended up “basically incinerated,” with nothing left but the engine.

Debris was scattered across a dozen checkpoint booths. The governor, a Democrat, called video of the airborne car “absolutely surreal".

Law enforcement personnel block off the entrance to the Rainbow Bridge in Niagara Falls, New York. – AP

Photos and video taken by bystanders and posted on social media showed thick smoke, flames on the pavement and a security booth that had been singed.

A Customs and Border Protection worker in a checkpoint booth was treated at a hospital for minor injuries and released, Hochul said.

From inside Niagara Falls State Park, Melissa Raffalow said she saw “a huge plume of black smoke” rise up over the border crossing, roughly 50 yards (45 meters) away from the popular tourist destination.

Raffalow said police arrived soon after, urging visitors to disperse as they began cordoning off the street.

Raghu Bhattarai said that he was inside his restaurant, the Niagara Tandoori Hut, near the bridge when he heard a sound he described as a “boom”. A few minutes later, he saw black smoke rising.

The Rainbow Bridge – a short span that offers striking views of the falls – and three others between western New York and Ontario were quickly closed as a precaution, though the other three later reopened.

The Buffalo-Niagara International Airport began security checks on all cars and told passengers to expect additional screenings.

The safety measures tied up traffic at the airport and elsewhere on one of the busiest US travel days of the year, ahead of the American Thanksgiving holiday.

In Toronto, about 100 miles (about 160km) away, police said they were increasing patrols as a precaution.

New York City police were monitoring the news from Buffalo but already had boosted security at various spots because of the Macy's Thanksgiving Day Parade.

About 6000 vehicles cross the Rainbow Bridge each day, according to the US Federal Highway Administration’s National Bridge Inventory. About 5 per cent is truck traffic, according to the federal data.

The bridge, constructed in 1941, is just over 1440 feet (439 meters) long and has a main span constructed of steel, according to the data.