A large balloon drifts above the Atlantic Ocean, just off the coast of South Carolina, on Saturday. AP

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, Gen. 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.

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

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.

Sailors recover a high-altitude surveillance balloon off the coast of Myrtle Beach, South Carolina. AP

VanHerck said the US adjusted its radar so it could track slower objects. "With some adjustments, we’ve been able to get a better categorization 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."

Asked if officials have ruled out extraterrestrials, 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.

The extraordinary air defense 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. Since, then 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 Sunday hovering over the Upper Peninsula of Michigan and it was going over Lake Huron, Pentagon officials said Sunday.

US and Canadian authorities had restricted some airspace over the lake earlier 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 discernable payload. It was flying low at about 20,000 feet, said the official who spoke to the media 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.

Associated Press