A vehicle exploded at a checkpoint on the American side of a US-Canada bridge in Niagara Falls on Wednesday, leaving two people dead and prompting the closing of four border crossings in the area, authorities said.

There was no immediate information on the cause of the explosion but it raised concerns on both sides of the border.

Canadian Prime Minister Justin Trudeau said officials were "taking this extraordinarily seriously", and the White House said President Joe Biden was "closely following developments".

The two deceased people were in the vehicle, a law enforcement official told AP. The official was not authorised to discuss details of the investigation publicly and spoke on condition of anonymity.

"This is obviously a very serious situation in Niagara Falls," Trudeau said in Parliament before excusing himself from Question Period in the House of Commons to be briefed further.

The US FBI's field office in Buffalo was investigating the blast, along with other agencies. New York Governor Kathy Hochul was travelling to Buffalo from the state capital, Albany.

The blast happened on the US side of the Rainbow Bridge, which connects the two countries across the Niagara River.

Photos and videos taken by bystanders and posted on social media showed thick smoke, flames on the pavement and a security booth that had been singed. Videos showed that the fire was in a US Customs and Border Protection area just east of the main vehicle checkpoint. The agency had no immediate comment.

Speaking to WGRZ-TV, witness Mike Guenther said he saw a vehicle speeding toward the crossing from the US side 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 30 or 40 feet high," Guenther told the station. "I never saw anything like it."

The Niagara Falls Bridge Commission called the incident "a serious car crash".

The bridge and three others between western New York and Ontario were quickly closed as a precaution, and the Buffalo-Niagara International Airport began security checks on all cars and told passengers to expect additional screenings.

Trudeau said "additional measures" were being contemplated and activated at border crossings across Canada. And in Toronto, about 100 miles (about 160 kilometres) away, police said they were increasing patrols as a precaution.

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 metres) away from the popular tourist destination. Raffalow told AP in a message that police arrived soon after, urging visitors to disperse as they began cordoning off the street.

Raghu Bhattarai said by phone 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 Niagara Falls Bridge Commission reported that all four of its crossings -- the others are Lewiston, Whirlpool and Peace Bridge -- were closed.

About 6,000 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 1,440 feet (439 metres) long and has a main span constructed of steel, according to the data.