The US military said it shot down a high-altitude object over Lake Huron on Sunday, in the fourth operation of its kind this month over the skies of North America. The mission by US fighter jets off the shores of Michigan and close to the Canadian border came after they separately shot down unidentified aerial objects over Yukon, the northwestern Canadian territory, on Saturday, and Alaska on Friday.

 On February 4, the US military shot down a suspected Chinese surveillance balloon over the Atlantic off the coast of South Carolina after it crossed expansive parts of Canada and the US. US officials have been trying to identify the origins and purpose of the three aerial objects shot down in the past three days, but so far have not announced any conclusions. The incidents are raising pressure on the Biden administration to better protect US airspace and assess the risks to civilian populations and national security. “What’s gone on the last two weeks or so, 10 days, 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 that go with it,” Jon Tester, the Democratic senator from Montana, told CBS on Sunday morning.

 North American Aerospace Defense Command said an F-16 fighter jet shot down “an airborne object” that was flying at roughly 20,000 feet over Lake Huron in Michigan. It added that indications were that the object was conducting surveillance on sensitive US military sites. “Based on its flight path and data we can reasonably connect this object to the radar signal picked up over Montana, which flew in proximity to sensitive Department of Defence sites,” the Pentagon said in a statement. “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.”

 Earlier in the day, Chuck Schumer, the Senate majority leader, said he had been briefed by Jake Sullivan, the US national security adviser, on Saturday night and said the two objects shot down at 40,000 feet on Friday and Saturday were believed to be balloons, but “much smaller than the first one.” Later, a National Security Council spokesperson said: “These objects did not closely resemble, and were much smaller than, the PRC [People’s Republic of China] balloon and we will not definitively characterise them until we can recover the debris, which we are working on.” Justin Trudeau, Canada’s prime minister, said Canadian teams were working to recover the object brought down on Saturday, which officials described as “cylindrical” in shape. “There’s still much to know about it. That’s why the analysis of this object is going to be very important,” Trudeau said. The Chinese balloon incident has already led to a deterioration in US-China relations, which had sunk to their lowest level since the two countries established diplomatic relations in 1979. Secretary of state Antony Blinken cancelled a trip to China at the last minute during which he was to meet President Xi Jinping. China has criticised the US for downing the balloon, which it says was a civilian vessel conducting meteorological research. Schumer called for China to cancel the balloon programme. “I think the Chinese were humiliated. I think the Chinese were caught lying. And I think it’s a real step back for them,” the senator said. “I think they’re probably going to have to get rid of it or do something, because they look really bad. And they’re not just doing the United States.” Some Democrats were uneasy that the Biden administration was not more forthcoming with information. “I sort of see a pattern as I looked at social media this morning. All of a sudden, massive speculation about alien invasions and, you know, additional Chinese actions or Russian action,” Jim Himes, a Connecticut Democrat, told NBC.

 “In the absence of information, people’s anxiety leads them into potentially destructive areas. So, I do hope that very soon the administration has a lot more information for all of us on what’s going on.” The episodes have also triggered a debate about whether the US military needs to invest more to improve its capacity to detect objects. Mike Turner, chair of the House Intelligence committee and an Ohio Republican, told CNN the US lacked “adequate radar systems” and “integrated missile defence” systems. “We really have to declare that we’re going to defend our airspace and then we need to invest.” The Biden administration has said the Chinese balloon entered US airspace over Alaska on January 28. But officials have not made clear if the US detected the object earlier, raising questions about the military’s aerial awareness capabilities. General Glen VanHerck, the US officer who heads North American Aerospace Defense Command, revealed this week that the military had not detected four previous Chinese balloon flights over the US — three during the Trump administration and one early under Biden — in real time.