A US fighter jet on Saturday shot down a Chinese spy balloon off the coast of South Carolina, the Pentagon said, over what it called Beijing's "unacceptable violation" of American sovereignty.

The craft spent several days flying over North America, ratcheting up tensions between Washington and Beijing, before it was targeted with a missile shot from an F-22 plane, Pentagon officials said, falling into relatively shallow water just 14 metres deep.

US Secretary of Defence Lloyd Austin called the operation a "deliberate and lawful action" that came in response to China's "unacceptable violation of our sovereignty."

Saturday afternoon was the military's first chance to take on the balloon "in a way that would not pose a threat to the safety of Americans," a senior defence official told reporters, while still allowing authorities to collect the fallen debris from US territorial waters.

In eyewitness video posted to social media, the balloon appeared to disintegrate in a white puff before its remnants dropped vertically into the ocean below.

Twitter user Haley Walsh posted that she "heard and felt the explosion" in Myrtle Beach, a popular resort town in South Carolina.

President Joe Biden, who earlier Saturday had promised "to take care" of the balloon, congratulated the fighter pilots involved.

"They successfully took it down. And I want to compliment our aviators who did it," Biden told reporters in Maryland.

'Force majeure'

The controversy erupted Thursday when American officials said they were tracking a large Chinese "surveillance balloon" in US skies.

That led Secretary of State Antony Blinken on Friday to scrap a rare trip to Beijing designed to contain rising US-China tensions.

After initial hesitation, Beijing admitted ownership of the "airship," but said it was a weather balloon that had been blown off course.

"The airship is from China. It is a civilian airship used for research, mainly meteorological, purposes," China's foreign ministry said in a statement Friday.

"The Chinese side regrets the unintended entry of the airship into US airspace due to force majeure," it said, using the legal term for an act outside of human control.

The balloon first entered US airspace over Alaska on January 28, Pentagon officials told reporters Saturday, before drifting over Canada and then back into the United States days later.

It was not the first time in recent history such an aircraft had flown over US territory, the senior defence official said, though this was the longest time one had spent in the country -- three balloons were spotted during Donald Trump's presidency and another one earlier in the Biden administration.

Biden told reporters he had on Wednesday ordered the craft shot down "as soon as possible."

"They decided -- without doing damage to anyone on the ground... that the best time to do that was as it got over water," Biden said.

According to the senior defence official, the military determined the airship was not a major threat to the United States during its flight, and "the surveillance balloon's overflight of US territory was of intelligence value to us," he added, without providing details.

Balloons across five continents

Teams were already working on recovering the balloon's remains, a senior military official said Saturday.

The balloon had flown over parts of the northwestern United States, including the state of Montana, that are home to sensitive airbases and strategic nuclear missiles in underground silos.

"We are confident it was seeking to monitor sensitive military sites," the senior defence official said.

Canadian Prime Minister Justin Trudeau also praised the operation, tweeting, "Canada strongly supports this action -- we'll keep working together... on our security and defence."

Republican lawmakers had quickly pounced on the balloon incident, casting Biden -- who has largely preserved, and at times expanded, Trump's hawkish policies on China -- as weak.

Top Democratic Senator Chuck Schumer tweeted Saturday that US experts now "can collect the equipment and analyze the technology used by the CCP," referring to the Chinese Communist Party.

By late Saturday afternoon, the Federal Aviation Administration had opened the airspace off the coast of the Carolinas, after three southeastern airports were temporarily shut down over a "national security effort.

After the balloon was spotted over North and South Carolina Saturday morning, a sheriff in York County, South Carolina, warned locals not to take matters into their own hands.

"It's flying at 60,000+ feet. Don't try to shoot it!! Your rifle rounds WILL NOT reach it," Sheriff Kevin Tolson tweeted.

Another suspected Chinese spy balloon was seen over Latin America, the Pentagon said Friday, without providing details.

"Over the past several years, Chinese balloons have previously been spotted over countries across five continents, including East Asia, South Asia and Europe," the senior defence official said Saturday.