The US military shot down a flying object over Lake Huron near the US-Canada border, the Pentagon said on Sunday, in the fourth such interception by American fighters this month.
Air Force Gen Glen VanHerck, who is tasked with safeguarding US airspace, told reporters that the military has not been able to identify what the three most recent objects are, how they stay aloft, or where they are coming from.
“We're calling them objects, not balloons, for a reason,” said Gen VanHerck, head of North American Aerospace Defense Command (NORAD) and Northern Command.
Asked if he's ruled out extraterrestrials, Gen VanHerck said he had not ruled out anything yet. “I'll let the intel community and the counter-intelligence community figure that out,” he said.
Another defence official, speaking on condition of anonymity, subsequently said the military had not seen any evidence that the objects were extraterrestrial.
Meanwhile, Canadian investigators are hunting for the wreckage of an unidentified flying object that was shot down by a US jet over Yukon territory on Saturday.
“Recovery teams are on the ground, looking to find and analyse the object,” Prime Minister Justin Trudeau said on Sunday.
“The security of citizens is our top priority and that's why I made the decision to have that unidentified object shot down.”
Mr Trudeau said it had posed a danger to civilian aircraft.
North America has been on high alert for aerial intrusions after the appearance of a white Chinese balloon over American skies this month.
The 60-metre tall balloon — which Americans have accused Beijing of using to spy on the US — caused an international incident, leading Secretary of State Antony Blinken to call off a planned trip to China hours before he was set to depart.
Surveillance fears appear to have US officials on high alert.
Twice in 24 hours, US officials closed airspace only to reopen it swiftly. On Sunday, the Federal Aviation Administration briefly closed space above Lake Michigan.
On Saturday, the US military scrambled fighter jets in Montana to investigate a radar anomaly there.
US Representative Elissa Slotkin, a Democrat who represents a district in Michigan, said the military had an “extremely close eye” on an object above Lake Huron, which is east of Lake Michigan on the US-Canada border.
Canada also closed airspace on Sunday near Tobermory, Ontario, which is on Lake Huron near the US border, according to Nav Canada, a private non-profit that operates Canada's air traffic control system.
China denies the first balloon was being used for surveillance and says it was a civilian research craft. It condemned the US for shooting it down off the coast of South Carolina last Saturday.
With military and intelligence officials newly focused on airborne threats, at least two other flying objects have since been destroyed over North America.
US Senate majority leader Chuck Schumer told US broadcaster ABC that us officials think the two latest objects were also balloons.
The original balloon was brought down off the coast of South Carolina on February 4.
US Navy recovers Chinese balloon — in pictures
A second was shot down over sea ice near Deadhorse, Alaska, on Friday. The third was destroyed over the Yukon on Saturday.
“They believe they were [balloons], yes, but much smaller than the first one,” Mr Schumer said.
The White House said only that the recently downed objects “did not closely resemble” the Chinese balloon, echoing Mr Schumer's description of them as “much smaller.”
Mr Schumer said he was confident US investigators scouring the ocean off South Carolina to recover debris and electronics from the original balloon would find out what it was being used for.
Canadians trying to piece together what was shot down over the Yukon may have their own challenges. The territory is a sparsely populated region in Canada's far north-west, which borders Alaska.
It can be brutally cold in the winter, but temperatures are unusually mild for this time of year, which could ease the recovery effort.
House of Representatives foreign affairs committee chairman Michael McCaul told Fox News the balloon shot down over the South Carolina coast had been on a mission to get imagery of sensitive American nuclear sites.
“They want to get imagery, get intelligence on our military capability, particularly nuclear,” Mr McCaul said. “And they're building quite a nuclear stockpile themselves.”
Republican Mike Turner, who is on the US House armed services committee, suggested the White House might be overcompensating for what he described as its previously lax monitoring of American airspace.
“They do appear somewhat trigger-happy,” Mr Turner told CNN on Sunday. “I would prefer them to be trigger-happy than to be permissive.”
Republicans have criticised the Biden administration over its handling of the incursion by the suspected Chinese spy balloon, saying it should have been shot down much earlier.
— Agencies contributed to this report