(AFP) - A US warplane shot down a new flying object near the Canadian border on Sunday, the latest of three mysterious devices spotted since military radar settings were cranked up in the wake of the downing of a suspected Chinese spy balloon.

Jittery Americans have been watching the skies as the mysterious incursions unfolded against a backdrop of acute tensions with China -- although only the first object has so far been attributed to Beijing.

On Sunday the Pentagon said that it does not yet know what the other three objects -- one shot down Friday over Alaska, one Saturday over Canada's Yukon territory, and the most recent one Sunday over Lake Huron -- are.

But it said that the object downed Sunday had been tracked for nearly a day and did not resemble the alleged Chinese surveillance balloon that was destroyed off the Atlantic coast on February 4 after traversing the country.

President Joe Biden ordered a F-16 fighter to shoot down the latest object "out of abundance of caution," a senior administration official said.

The object was described by the official as an octagonal structure with strings hanging off it.

Drifting at about 20,000 feet (6,000 meters) over Michigan, it could have posed a hazard to civil aviation, the official said.

US Northern Command Commander General Glen VanHerck told reporters that after aircraft were sent up to inspect the newest object, they concluded that there was no indication of any threat, the same with the previous objects.

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

He declined to describe the shape or size of the objects, but said they were traveling very slowly, around the speed of the wind.

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

"I will let the intel community and the counterintelligence community figure that out," VanHerck told reporters when asked if it was possible the objects are aliens or extra-terrestrials.

"I haven't ruled out anything at this point."

- Mysterious objects -

The objects shot down since Friday were detected after US air defense adjusted radar settings to scan for smaller and slower-moving objects, said Assistant Secretary of Defense Melissa Dalton.

"In light of the People's Republic of China balloon that we took down last Saturday, we have been more closely scrutinising our airspace at these altitudes, including enhancing our radar, which may at least partly explain the increase in objects that we've detected over the past week," she told reporters.

She said they are aware that there are objects drifting at such altitudes operated in the air by research institutes and private companies.

"But because we had not been able to definitively assess what these recent objects are, the president wanted to act out of an abundance of caution to protect our security and interests," she said.

Currently operations are underway to recover all four objects, said Dalton.

Canadian officials described the shot down over Canada's far northwest on Saturday as small and cylindrical, roughly the size of a Volkswagen car.

Recovery teams backed by a Canadian CP-140 patrol aircraft were continuing their search Sunday for debris in the Yukon, officials said.

US teams were struggling with Arctic conditions as they searched near Deadhorse, Alaska, where the second object was shot down Friday.

Operations were also continuing off the South Carolina coast, where the past week's drama climaxed when the initial large balloon was shot down.

- China relations tense -

China has insisted that first object was a weather balloon blown off course.

The incident prompted Secretary of State Antony Blinken to cancel a long-planned diplomatic mission to Beijing.

A senior Republican on Sunday accused Beijing of "an act of belligerence."

"It was done with provocation to gather intelligence data, and collect intelligence on our three major nuclear sites," Michael McCaul, chairman of the House Foreign Affairs Committee, told CBS.

After being rebuffed by Beijing for days, Dalton said US officials have now communicated with Beijing on the alleged spy balloon.

"There have been contacts made with the PRC on the high altitude balloon," she said.

US Defense Secretary Lloyd Austin asked to speak with Chinese counterparts shortly after the balloon was shot down.

China's Defense Ministry said Thursday it had refused the offer of a call because of Washington's "irresponsible" decision to shoot down the balloon.

© Agence France-Presse