US President Joe Biden ordered the military to shoot down another unidentified flying object above Lake Huron, near the border with Canada, on Sunday.

"We did not assess it to be a kinetic military threat to anything on the ground, but assess it was a safety flight hazard and a threat due to its potential surveillance capabilities," the Department of Defense said.

The object, which had an octagonal shape with no discernible payload, was shot down out of an "abundance of caution," a senior US official added.

It is the third unidentified object to be shot down by American fighter jets in as many days, after similar incidents in Alaska and Canada.

"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 from the state of Michigan, which lies south of the lake.

Tension over what the US sees as attempted spying peaked after a massive Chinese balloon was sighted above the US in late January. China insisted the balloon was a "civilian airship used for research," while the US described it as a "spy balloon" and shot it down

Senior US General does not rule out aliens

The US Air Force General overseeing the North American airspace said that he would not rule out aliens or any other explanation just yet. 

"I'll let the intel community and the counterintelligence community figure that out," said General Glen VanHerck during a press briefing. 

Meanwhile, another US defense official who spoke on the condition of anonymity said that the military had seen no evidence which suggested that the objects in question were of extraterrestrial origin. 

Downed UFOs 'much smaller' than Chinese balloon — White House

Assistant Secretary of Defense Melissa Dalton said three unidentified objects being spotted within the space of days was likely due to the way the military has been scanning the skies.

"In light of the People's Republic of China balloon that we took down last Saturday, we have been more closely scrutinizing 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 on Sunday.

Earlier on Sunday, a White House spokesperson said the flying objects which the US shot down over Alaska and Canada in the last few days were much smaller than the Chinese balloon which was downed a week ago.

On Saturday, US fighter jets shot down an object flying over Yukon near the US border with Canada, after shooting down another flying object a day before near Deadhorse, Alaska.

"These objects did not closely resemble and were much smaller than the PRC balloon and we will not definitively characterize them until we can recover the debris, which we are working on," the Reuters news agency quoted the spokesperson as saying.

Canadian Prime Minister Justin Trudeau said one of the objects had violated Canadian airspace. He added in a tweet late on Saturday that Canadian forces will recover and analyze the wreckage.

What do we know about the flying objects?

The Canadian Prime Minister said the unidentified object was brought down by a US F-22, as per his orders.

"Canadian Forces will now recover and analyze the wreckage of the object," Trudeau said, adding that he was in touch with US President Joe Biden.

Earlier on Sunday, Senate Majority Leader Chuck Schumer told the ABC broadcaster that the shot down objects were balloons, though smaller than the Chinese "spy balloon."

Schumer said he had been briefed the night before by Biden's national security advisor, Jake Sullivan. He added that the two objects also flew at a lower altitude of about 40,000 feet (around 12,200 meters).

The surveillance balloon had flown at about 60,000 feet.

The North American Aerospace Defense Command (NORAD) said on Sunday they had temporarily restricted flights over the Lake Michigan area. 

The restriction was carried out to "ensure the safety of air traffic in the area during NORAD operations," it said, without giving details on the nature of the operations.

The measure has since been lifted.

The sight of the Chinese balloon drifting over the US prompted to a political uproar in Washington, bringing into focus the challenges China posed to the US and its allies.

zc, rmt/dj, jsi (AP, dpa, Reuters, AFP)