US terrorism investigators deployed Wednesday after a car erupted into a fireball at a US-Canada checkpoint, triggering border closures on one of the busiest travel days in the American holiday calendar

Two people were killed in the blast, according to US media citing authorities, although their identities were not yet public.

The car exploded at the major Rainbow Bridge crossing near Niagara Falls, local and state authorities said.

The incident's cause was not immediately clear and it was also not confirmed in which direction the car was crossing -- into Canada or the United States.

However, authorities on both sides of the border reacted urgently.

The White House said that President Joe Biden had been briefed.

Canadian Prime Minister Justin Trudeau told parliament "this is obviously a very serious situation" and he announced closure of four crossings.

New York Governor Kathy Hochul said state police and the FBI Joint Terrorism Task Force were monitoring all points of entry into the state.

US media cited law enforcement sources as saying there were two people in the car, which had passed through one border checkpoint where it was selected for a secondary check. 

The car then sped up, crashed into a barrier and exploded, the sources said, stressing it was unclear if the blast was the result of a device.

Mike Guenther, a Canadian visiting the United States, described a dramatic, high-speed crash at the checkpoint.

"We’ve seen this car coming down towards the border and he was flying – over 100 miles an hour," he told CBS News.

The car then swerved and "hit the fence, went flying up into the air," he said. "He went up into the air and we just seen the fireball and that's all we could see. It was just covered in smoke everywhere."

The incident came on the eve of the Thanksgiving holiday, one of the busiest days for travel on the US holiday calendar when millions of Americans take to the roads and skies.

Rainbow Bridge -- among the busiest crossings between Canada and the United States -- has 16 vehicle lanes and is normally open around the clock, according to US Customs and Border Protection.

- 'Violent circumstance' -

Canada's Public Safety Minister Dominic LeBlanc said "any time... (infrastructure like) a border crossing sees this kind of violent circumstance, it's a source of concern for" Washington and Ottawa.

Hochul announced that she was traveling to Buffalo to meet with law enforcement and emergency responders and would update New Yorkers when more information became available.

Trudeau's office said he had been briefed and was in contact with US law enforcement. Multiple Canadian agencies were supporting the investigation, it added.

Multiple witnesses reported hearing a loud explosion and seeing a large cloud of smoke near the inspections station.

Ivan Vitalii, a visitor from Ukraine, told the Niagara Gazette he and a friend were at a nearby convenience store on the US side when they saw a car exit a parking lot and travel toward the bridge.

"We heard something smash," he said. "We saw fire and big, black smoke."

Dramatic images posted to social media, which AFP has not verified, showed thick plumes of black smoke rising from a conflagration at the crossing.

burs-gw/sms