Canada has closed the section of airspace over the Great Lakes and near the US-Canada borders citing an “Active Air Defense Operation.”

US officials restricted the airspace over Lake Michigan, one of the Great Lakes, on Sunday due to a potential new threat to national security but reopened the skies, as the US and Canada respond to multiple air intrusions.

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The Federal Aviation Administration announced temporary flight restrictions over the Great Lakes along the US-Canada border, citing it as “national defense airspace”. The restrictions were lifted shortly after they were announced, apparently with no threat found.

“The FAA briefly closed some airspace over Lake Michigan to support Department of Defense activities. The airspace has been reopened,” the agency said in a statement, reported AFP.

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The Lake Michigan closure marked the latest move to address a series of potential national security threats that started in late January with the detection of a Chinese balloon, called spycraft by US authorities, hovering over the United States at a high altitude.

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This is after the US Federal Aviation Administration (FAA) had temporarily closed airspace in Havre, Montana airspace and near the US-Canada border to support US military operations. The airspace was reopened later Saturday night. “The FAA has closed airspace in Montana to support Department of Defense operations. Airspace has been reopened,” the FAA said in a statement.

The Montana closure was ordered over a “radar anomaly,” but no object was discovered.

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On Saturday night, a mystery object was seen flying in the skies above the areas 30 miles south of the Canadian border. Canadian Prime Minister Justin Trudeau said “an unidentified object” that violated Canadian airspace had been shot down by U.S. F-22s in Yukon by US and Canadian forces in a joint operation. It was a joint order given by Trudeau and Biden, the White House states later.