The US military said late on Saturday that a radar anomaly had prompted the temporary closure of airspace to civilian airplanes over Montana but that no threatening object was detected.

The North American Aerospace Defense Command (NORAD) sent fighter aircraft to investigate but reported the aircraft "did not identify any object to correlate to the radar hits." It said it would continue to monitor the situation.

Earlier on Saturday, a US F-22 fighter jet shot down an unidentified cylindrical object over Canada, the second such shootdown in as many days. Canada and the United States have been on heightened alert following an episode earlier this month where a high-altitude balloon originating from China was tracked from Montana to South Carolina and then shot down off the coast.

Three lawmakers said on Twitter that an unidentified object was seen in Montana airspace. Representative Matt Rosendale, a Montana Republican, said on Twitter he was in contact with the US military "and monitoring the latest issue over Havre [in Montana] and the northern border."

Canadian Prime Minister Justin Trudeau was the first to announce a shootdown over the northern Yukon territory, saying Canadian forces would recover and analyse the wreckage.

In a media conference, Canadian Defence Minister Anita Anand declined to speculate about the origin of the object, which was aloft at an altitude of about 40,000 feet.

"There is no reason to believe that the impact of the object in Canadian territory is of any public concern," Anand said.

The Pentagon said US President Joe Biden had authorised the US military to work with Canada to take down the craft after a call between Biden and Trudeau.

A day earlier, Biden ordered another shootdown of an unidentified flying object near Deadhorse, Alaska. (Reuters)