US-Canada border crossings near Niagara Falls were closed Wednesday as terrorism investigators responded to a vehicle explosion at Rainbow Bridge, local and state authorities said

The cause of the incident was not immediately clear but New York Governor Kathy Hochul said state police and the FBI Joint Terrorism Task Force were monitoring all points of entry into the state.

"I am traveling to Buffalo to meet with law enforcement and emergency responders and will update New Yorkers when more information becomes available," she posted on social media.

The FBI Buffalo field office said in a statement it was "investigating a vehicle explosion" at the bridge and added that the situation was "very fluid, that's all we can say at this time."

Ron Rienas, of the Buffalo and Fort Erie Public Bridge Authority, told ABC News all four bridges connecting the two countries over the Niagara River have been closed.

Pictures posted to social media, which AFP has not verified, purported to show thick plumes of black smoke rising from a large conflagration at the crossing.

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