It was the fourth such downing in eight days and the latest military strike in an extraordinary chain of events over US airspace that Pentagon officials believe has no peacetime precedent.

Part of the reason for the repeated shootdowns is a “heightened alert” following a spy balloon from China that emerged over US airspace in late January, General Glen VanHerck, head of Norad and US Northern Command, said in a briefing with reporters.

Since then, fighter jets last week also shot down objects over Canada and Alaska. Pentagon officials said they posed no security threats, but so little was known about them that Pentagon officials were ruling nothing out — not even UFOs.

A US fighter jet shot down an “unidentified object” over Lake Huron on Sunday on orders from President Joe Biden, with military officials saying they are unsure how three of the unidentified flying objects had been able to stay aloft.

“We have been more closely scrutinising our airspace at these altitudes, including enhancing our radar, which may at least partly explain the increase,” said Melissa Dalton, assistant defence secretary for homeland defence.

US authorities have made clear that they constantly monitor for unknown radar blips, and it is not unusual to shut down airspace as a precaution to evaluate them.

But the unusually assertive response was raising questions about whether such use of force was warranted, particularly as administration officials said the objects were not of great national security concern and the downings were just out of caution.

Mr VanHerck said the US adjusted its radar so it could track slower objects.

“With some adjustments, we’ve been able to get a better categorisation of radar tracks now,” he said, “and that’s why I think you’re seeing these, plus there’s a heightened alert to look for this information.”

He added: “I believe this is the first time within United States or American airspace that Norad or United States Northern Command has taken kinetic action against an airborne object.”

The US Northern Command Commander General added that there was no indication of any threat.

"I'm not going to categorise them as balloons. We're calling them objects for a reason.

"What we are seeing is very, very small objects that produce a very, very low radar cross-section.

Speculation as to what the objects may be has intensified in recent days.

Asked if officials have ruled out extra-terrestrials, Mr VanHerck said: “I haven’t ruled out anything at this point.”

The Pentagon officials said they were still trying to determine what exactly the objects were and said they had considered using the jets’ guns instead of missiles, but it proved to be too difficult. They drew a strong distinction between the three shot down over this weekend and the balloon from China.

Minnesota Governor Tim Walz tweeted that airmen in the 148th Fighter Wing, an Air National Guard fighter unit in Duluth, shot down the object over Lake Huron.

The extraordinary air defence activity began in late January, when a white orb the officials said was from China appeared over the US and hovered above the nation for days before fighter jets downed it off the coast of Myrtle Beach, South Carolina.

That event played out over livestream. Many Americans have been captivated by the drama playing out in the skies as fighter jets scramble to shoot down objects.

The latest brought down was first detected on Saturday evening over Montana, but it was initially thought to be an anomaly. Radar picked it up again on Sunday hovering over the Upper Peninsula of Michigan and it was going over Lake Huron, Pentagon officials said.

US and Canadian authorities had restricted some airspace over the lake earlier on Sunday as planes were scrambled to intercept and try to identify the object.

According to a senior administration official, the object was octagonal, with strings hanging off, but had no discernible payload. It was flying low at about 20,000 feet, said the official who spoke to The Associated Press on condition of anonymity to discuss sensitive matters.

Meanwhile, US officials were still trying to precisely identify two other objects shot down by F-22 fighter jets, and were working to determine whether China was responsible as concerns escalated about what Washington said was Beijing’s large-scale aerial surveillance programme.

An object shot down on Saturday over Canada’s Yukon was described by US officials as a balloon significantly smaller than the balloon — the size of three school buses — hit by a missile on February 4.

A flying object brought down over the remote northern coast of Alaska on Friday was more cylindrical and described as a type of airship.

Both were believed to have a payload, either attached or suspended from them, according to the officials who spoke to The Associated Press on condition of anonymity to discuss the ongoing investigation. Officials were not able to say who launched the objects and were seeking to figure out their origin.

The three objects were much smaller in size, different in appearance and flew at lower altitudes than the suspected spy balloon that fell into the Atlantic Ocean after the US missile strike.

The officials said the other three objects were not consistent with the fleet of Chinese aerial surveillance balloons that targeted more than 40 countries, stretching back at least into the Trump administration.

The cases have increased diplomatic tensions between the United States and China, raised questions about the extent of Beijing’s American surveillance, and prompted days of criticism from Republican lawmakers about the administration’s response.

Lake Huron: US military ‘unclear’ what fourth shot down ‘unidentified object’ is as they refuse to rule anything out

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