The US has shot down another unidentified flying object in the fourth military operation of its kind this month.

President Joe Biden ordered it to be downed near Lake Huron, close to the Canadian border, on Sunday afternoon.

The object could have interfered with commercial air traffic as it was traveling at 20,000ft (6,100m), a Pentagon statement said.

It was first detected above military sites in Montana on Saturday, it added.

The object, which was not deemed a military threat, has been described by defence officials as an unmanned “octagonal structure” with strings attached to it. It was downed by a missile fired from an F-16 fighter jet at 14:42 local time (19:42 GMT).

The incident raises further questions about the spate of high-altitude objects that have been shot down over North America this month.

A suspected Chinese spy balloon was downed off the coast of South Carolina on 4 February after hovering for days over the continental US. Officials said it originated in China and had been used to monitor sensitive sites.

China denied the object was used for spying and said it was a weather monitoring device that had been blown astray. The incident - and the angry exchanges in its aftermath - ratcheted up tensions between Washington and Beijing.

But on Sunday, a defence official said the US had communicated with Beijing about the first object after receiving no response for several days. It was not immediately clear what was discussed.

Since that first incident, American fighter jets have shot down three further high-altitude objects in as many days.

President Biden ordered an object to be shot down over northern Alaska on Friday, and on Saturday a similar object was shot down over the Yukon in north-western Canada.

Officials have not publicly identified the origin or purpose of these objects. Both the US and Canada are still working to recover the remnants, but the search in Alaska has been hampered by Arctic conditions.

“These objects did not closely resemble, and were much smaller than, the [4 February] balloon and we will not definitively characterise them until we can recover the debris,” a White House National Security spokesperson said.

Unidentified flying objects - timeline

4 February: US military shoots down suspected surveillance balloon off the coast of South Carolina. It had drifted for days over the US, and officials said it came from China and had been monitoring sensitive sites

10 February: US downs another object off northern Alaska which officials said lacked any system of propulsion or control

11 February: An American fighter jet shoots down a “high-altitude airborne object” over Canada's Yukon territory, about 100 miles (160 km) from the US border. It was described as cylindrical and smaller than the first balloon

12 February: US jets shoot down a fourth high-altitude object near Lake Huron “out of an abundance of caution”

Later on Sunday, the US Air Force general overseeing North American airspace said it still wasn't clear how the objects were being propelled through the air or where they were coming from.

He also said he had not ruled any explanation out - including extraterrestrial life. “I'll let the intel community and the counterintelligence community figure that out,” Gen Glen VanHerck told reporters when asked about the possibility.

One senior official told ABC News that the three most recent objects to be shot down were likely weather devices and not surveillance balloons.

But this was contradicted by the top Democrat in Congress, who earlier told the broadcaster that intelligence officials believed the objects were in fact surveillance balloons.

“They believe they were [balloons], yes,” Senate Majority Leader Chuck Schumer said, adding that they were “much smaller” than the first one shot down off the South Carolina coast.

Democrat Debbie Dingell, one of several Michigan members of Congress who applauded the military for downing the object over the state on Sunday, joined growing calls for the White House and defence officials to provide more information.

“We need the facts about where they are originating from, what their purpose is, and why their frequency is increasing,” she said.

Democratic Senator Jon Tester, who represents Montana, told the BBC's US partner CBS: “What's gone on the last two weeks or so... has been nothing short of craziness. And the military needs to have a plan to not only determine what's out there, but determine the dangers.”

Republicans have repeatedly criticised the Biden administration for its handling of the first suspected spy balloon, saying it should have been shot down far sooner.

Meanwhile, Defence Secretary Ben Wallace said the UK would conduct a security review following the recent incidents in the US and Canada. “This development is another sign of how the global threat picture is changing for the worse,” he said. (BBC)

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