On Wednesday a vehicle exploded at a checkpoint on the American side of a U.S.-Canada bridge in Niagara Falls. Two people died and the incident prompted 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."

"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.

In the U.S. the FBI's field office in Buffalo was investigating the explosion. New York Governor Kathy Hochul was on her way to Buffalo after leaving the state capital, Albany.

The blast happened on the U.S. side of the Rainbow Bridge, which connects the two countries across the Niagara River. 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. Videos showed that the fire was in a U.S. Customs and Border Protection area just east of the main vehicle checkpoint.

Speaking to a TV station a witness said he saw a vehicle speeding toward the crossing from the U.S. side when it swerved to avoid another car, crashed into a fence and exploded. "All of a sudden, it went up in the air and then it was a ball of fire 10 to 15 metres high."

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

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

A restaurant owner said that he was inside his restaurant, near the bridge when he heard an explosion. 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.

Every day about 6,000 vehicles cross the Rainbow Bridge each day. About 5% is truck traffic, according to US federal data.

The bridge was constructed in 1941, is just over 439 metres) long and has a main span constructed of steel.

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