Washington strenuously denied Beijing's charges as "false" and insisted it was China alone that has surreptitiously flown spy balloons, as tensions ramped up between the two superpowers.

The downing of an alleged Chinese airship off the coast of South Carolina earlier this month prompted a resolution in the US Congress condemning Beijing, while subsequent downings of other unidentified objects over North America in recent days have sparked widespread jitters and speculation as to their origins.

Only the first object has been officially attributed to China, with Beijing insisting it was a civilian craft that had blown off course.

On Monday, China sought to turn the tables on Washington, accusing it of having sent more than 10 balloons into Chinese airspace since January 2022.

"It's not uncommon as well for the United States to illegally enter the airspace of other countries," foreign ministry spokesman Wang Wenbin said during a briefing.

"Since last year alone, US balloons have illegally flown above China more than 10 times without any approval from Chinese authorities."

Asked how China responded to those alleged incursions, Wang said Beijing's "handling (of these incidents) was responsible and professional".

"If you want to know more about US high-altitude balloons illegally entering China's airspace, I suggest you refer to the US side," he added.

The White House knocked back the allegation.

"Any claim that the US government operates surveillance balloons over the PRC is false," National Security Council spokesperson Adrienne Watson said on Twitter.

"It is China that has a high-altitude surveillance balloon program for intelligence collection, that it has used to violate the sovereignty of the US and over 40 countries across 5 continents."

The State Department responded with a similar rejection. It called Beijing's accusation "the latest example of China scrambling to do damage control," and said the communist government "has failed to offer any credible explanations for its intrusion into our airspace."

Over the weekend, Chinese state-affiliated media reported that an unidentified flying object had been spotted off the country's east coast -- and that the military was preparing to shoot it down.

Beijing on Monday declined to comment on that report, referring journalists to the defence ministry, which did not respond to requests for comment from AFP.

- Watching the skies -

Americans have been watching the skies as a growing number of aerial incursions are reported.

On Sunday the Pentagon said it had not yet determined the nature of the other three objects -- one shot down Friday over Alaska, one on Saturday over Canada's Yukon territory, and the most recent one Sunday over Lake Huron.

But it said 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 over the United States.

"We're going to do everything we can to recover them," White House spokesman John Kirby told MSNBC Monday.

"And as soon as we do that, we'll learn more about them."

The initial balloon prompted Washington to cancel a rare trip to Beijing by Secretary of State Antony Blinken.

President Joe Biden ordered an F-16 fighter to shoot down the latest object on Sunday "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 metres) over Michigan, it could have posed a hazard to civil aviation, the official said.

The US Northern Command's 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 travelling slowly, around the speed of the wind.

Speculation as to what the objects may be has flared 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 extraterrestrial.

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