An explosion has been reported on the American side of the U.S.-Canada border at the Rainbow Bridge crossing. 

ABC Buffalo reports that the bridge is closed in both directions "due to an incident involving a vehicle coming into the U.S."

"A spokesperson for the Niagara Falls Mayor's Office said the closure will be in place until further notice and federal authorities are currently investigating the situation," the news outlet reported. 

It's not known at this time whether the explosion was an accident, terror attack, or something else. 

Video from the area shows a massive cloud of smoke in the vicinity of the bridge. Reporter Keean Bexte tweeted that "a vehicle containing an unknown number of people and contents exploded."

The FBI Buffalo Field Office is investigating the explosion at the border. 

 It appears to have been a massive blast: 

These photos appear to show the explosion scene, but this early in the investigation, PJ Media cannot confirm whether they are authentic. 

ABC News reports that "Law enforcement is placing K-9 officers on each of the international bridges in the area as a precautionary measure, according to sources."

This is a developing story. Check back for updates.