Four border crossings between the US and Canada were closed after a vehicle exploded at a checkpoint on a bridge near Niagara Falls, reportedly killing two people.

Kathy Hochul, the governor of New York, said there was “no indication of a terrorist attack” in the explosion which happened on the US side of the Rainbow Bridge, which connects the two countries across the Niagara River.

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

Chuck Schumer, the senior senator from New York, tweeted that the FBI believed two people had died in the explosion.

“I was just briefed by the FBI on the incident at Rainbow Bridge. Initial reports indicate the two people killed were in the car but nothing’s been determined on their identity or motive. They continue to investigate – law enforcement remains on heightened alert over Thanksgiving,” he wrote.

Justin Trudeau excused himself from question period in Canada’s House of Commons to be briefed further, saying: “This is obviously a very serious situation in Niagara Falls.”

“We are taking this extraordinarily seriously,” the Canadian prime minister added.

The Rainbow bridge and three other crossings at Lewiston, Whirlpool and Peace Bridge – were closed soon after the blast, although the other three were reopened later on Wednesday.

The White House said it was “closely monitoring the situation at the US-Canada border crossing”, and that law enforcement officials were on the scene and investigating.

Photos and video taken by news organizations and posted on social media showed a security booth that had been singed by flames.

Videos showed the fire was in a US Customs and Border Protection area just east of the main vehicle checkpoint.

Speaking to WGRZ-TV, Mike Guenther said he saw a vehicle speeding toward 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 30 or 40ft high,” Guenther told the station. “I never saw anything like it.”

Ivan Vitalii, a Ukrainian visiting Niagara Falls, 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.

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 (45m) 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.

Doug Ford, the premier of Ontario, which borders New York state, said: “Our provincial law enforcement is actively engaged in assessing the situation. They are working with local law enforcement and are providing support as required.”

About 6,000 vehicles cross the Rainbow Bridge each day, according to the US Federal Highway Administration’s National Bridge Inventory. About 5% is truck traffic, according to the federal data.