Washington: A US fighter jet has shot down an object, the size of a small car, flying at a high altitude over Alaska on the direction of President Joe Biden, the White House said on Friday.

According to Pentagon Press Secretary Brigadier General Pat Ryder, the object posed a "reasonable threat to civilian air traffic".

The object, whose origin is yet to be ascertained, has fallen off the coast of Alaska in frozen water and an effort is on to collect as much debris as possible, he told reporters at a news conference.

This came almost a week after the US shot down a Chinese spy balloon off the coast of South Carolina in the Atlantic Ocean. It had hovered over continental America for several days after entering the US airspace on January 30 in Montana.

China has acknowledged that the balloon was theirs but denied that it was for surveillance purposes rather for weather monitoring and that it had drifted off course.

About the high-altitude flying object shot down on Friday, National Security Council Coordinator for Strategic Communications John Kirby said, "The Department of Defense was tracking it over Alaska airspace in the last 24 hours. It was flying at an altitude of 40,000 feet and posed a reasonable threat to the safety of the civilian flight."

"Out of an abundance of caution and at the recommendation of the Pentagon, President Biden ordered the military to down the object, and they did. It came inside our territorial waters... those waters right now are frozen," he told reporters at a White House news conference.

Fighter aircraft assigned to US Northern Command took down the object, he added.

This object, Kirby said, was the size of a small car. It was over a very sparsely populated area, but also above water when the president ordered this down, Kirby added.

"We expect the debris field for this to be much smaller than the other one (Chinese spy balloon). We knew that the Chinese balloon we shot down last week was a surveillance asset capable of surveillance over sensitive military sites and that it had self-propulsion and manoeuvre capabilities."

"There's no indication that this one did," he said.

A couple of days earlier, the US had claimed that there have been four previous instances of Chinese surveillance balloons flying over US territories.

Washington has accused China of "violating the sovereignty" of not just the United States but of countries across five continents.

According to a report of The Washington Post, based on interviews with several anonymous defence and intelligence officials, China has operated a fleet of spy balloons targeting several countries including India and Japan.