An explosion on Wednesday involving a vehicle at Rainbow Bridge, a border crossing between the United States and Canada in the Niagara Falls region, caused the closure of that bridge as well as the other three border crossings in western New York, according to the authorities.

The cause of the explosion has not been determined. But the F.B.I.’s Buffalo field office said in a post on X, formerly known as Twitter, that the agency was coordinating with local, state and federal law enforcement in investigating the explosion.

“As the situation is very fluid, that’s all we can say at this time,” the post said.

Officials did not say if there were any injuries caused by the explosion.

Gov. Kathy Hochul of New York said she had been briefed on the incident and was closely monitoring the situation, adding that “state agencies are on site and ready to assist.”

The New York State Police are working with the F.B.I. to monitor all points of entry to New York, Ms. Hochul said.

In Canada, Prime Minister Justin Trudeau told the House of Commons, “We are taking this extremely seriously,” in brief comments before he left for an update on the situation.

Dominic LeBlanc, the nation’s public safety minister, declined to say whether the vehicle that exploded had come from Canada because the government law enforcement agencies had received contradictory information.

“This is a rapidly changing situation,” he told reporters. “All measures are being taken to ensure that people will be safe.”

He said that at the request of American officials, all four bridges between Canada and the United States in the Niagara area have been closed, including the Rainbow Bridge, Peace Bridge, Lewiston-Queenston Bridge and Whirlpool Bridge.

Remaining international crossings are open and on heightened alert status, according to a spokesman with the governor’s office, while airports and railway facilities in the area have increased security. There are also additional explosive detective dogs and police patrols, and additional screening is being deployed at Buffalo Niagara International Airport.

A video from the scene showed what appeared to be an inspection booth on the bridge with smoke billowing out of the top and debris surrounding it.

The streets in the tourist area of downtown Niagara Falls were nearly empty for about a quarter-mile, as city police, state police and National Guard troops blocked access to the Rainbow Bridge. At around 2:15 p.m., several emergency responders were still rushing to the bridge.

A few curious people were denied information by authorities and turned away.

Paul Lane contributed reporting from Niagara Falls.