The US Air Force general in charge of North American airspace says he cannot rule out aliens – or any other explanation – for the spate of mystery objects detected in US skies.

General Glen VanHerck spoke late on Sunday, after another unidentified object, the fourth in little more than a week, was shot down over North America by a US missile.

On President Joe Biden’s order, a US F-16 fighter shot down the object at 2.42 pm local time over Lake Huron on the US-Canada border, the Pentagon said.

Asked at a late night briefing whether he had ruled out space aliens, General VanHerck said: “I’ll let the intel community and the counterintelligence community figure that out. I haven’t ruled out anything.”

General VanHerck leads the US North American Aerospace Defence Command and Northern Command.

However, a Reuters reporter is also reporting an anonymous US defence official as saying there’s “no indication of aliens or extraterrestrial activity with these recent take downs”.

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General VanHerck was also quizzed about the physical characteristics of the mystery objects.

“I’m not going to categorise them as balloons. We’re calling them objects for a reason,” he said.

“I’m not able to categorise how they stay aloft. It could be a gaseous type of balloon inside a structure or it could be some type of a propulsion system. But clearly, they’re able to stay aloft.”

The Pentagon said the “octagonal-shaped” object shot down on Sunday posed no military threat. But it could have potentially interfered with domestic air traffic as it was travelling at 20,000 feet (6100 metres), and it might have had surveillance activities.

The object had strings hanging from it but no discernible payload, a US official who spoke on condition of anonymity said.

It was recently detected over Montana near sensitive military sites, prompting the closure of US airspace, the Pentagon said.

General VanHerck stressed that officials did not know which country the objects had come from, as distinct from the suspected Chinese surveillance balloon shot down by the US military off the coast of South Carolina last week.

“I would be hesitant and urge you not to attribute it to any specific country. We don’t know,” he said.

But the latest incident has raised questions about the spate of unusual objects in 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,” US Representative Debbie Dingell, one of several Michigan politicians who applauded the military for downing the object, said.

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.

“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,” he said, adding that it had posed a danger to civilian aircraft.

North America has been on high alert for aerial intrusions following the appearance of a 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. Secretary of State Antony Blinken called off a planned trip to China only hours before he was set to depart.

Surveillance fears appear to have US officials on high alert.

Twice in the past 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.

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.

US Senate Majority Leader Chuck Schumer told US broadcaster ABC that 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 Mr Schumer’s description of them as “much smaller”.

Canadian counterparts 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.

-with AAP