United States military fighter jets have shot down an octagonal object over Lake Huron, the Pentagon says, the latest incident since a suspected Chinese surveillance balloon put North American security forces on high alert.
It was the fourth unidentified flying object to be shot down over North America by a US missile in a little more than a week.
China's foreign ministry said it had no information on the latest three flying objects shot down by the US.
US Air Force General Glen VanHerck told reporters 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," VanHerck, head of North American Aerospace Defense Command (NORAD) and Northern Command, said.
VanHerck would not rule out aliens or any other explanation.
"I'll let the intel community and the counterintelligence community figure that out," he said.
Another defence official, speaking on condition of anonymity, said the military had not seen any evidence that the objects were extraterrestrial.
On President Joe Biden's order, a US F-16 fighter shot down the object at 2.42pm local time over Lake Huron on the US-Canada border, Pentagon spokesman Brigadier General Patrick Ryder said in a statement.
Although it did not pose a military threat, the object could have potentially interfered with domestic air traffic as it was travelling at 6100 metres and might have had surveillance capabilities, Ryder said.
The object appeared to be octagonal, with strings hanging off but no discernible payload, a US official speaking on condition of anonymity said.
The object was believed to be the same as one recently detected over Montana near sensitive military sites, prompting the closure of US airspace, the Pentagon said.
The military will try to recover the object downed over Lake Huron to learn more about it, VanHerck told reporters.
He said it likely fell into Canadian waters.
The incident raised questions about the spate of unusual objects that have appeared over North American skies in recent weeks and raised tensions with China.
"We need the facts about where they are originating from, what their purpose is and why their frequency is increasing," said US Representative Debbie Dingell.
US officials identified the first object as a Chinese surveillance balloon and shot it down off the coast of South Carolina on February 4.
On Friday, a second object was shot down over sea ice near Deadhorse, Alaska.
A third object was destroyed over Canada's Yukon on Saturday, with investigators still hunting for the wreckage.
"The security of citizens is our top priority and that's why I made the decision to have that unidentified object shot down," Prime Minister Justin Trudeau told reporters on Sunday.
North America has been on guard against aerial intrusions following the appearance of the white, eye-catching Chinese airship over American skies earlier this month.
That 60-metre-high 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.
Pentagon officials said they have been scrutinising radar more closely since then.
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.
US Senate Majority Leader Chuck Schumer told US broadcaster ABC officials think two of the latest objects were smaller balloons than the original one.
The White House said only that the recently downed objects "did not closely resemble" the Chinese balloon, echoing Schumer's description of them as "much smaller".
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