WASHINGTON — A U.S. military fighter jet shot down an unknown object flying off the coast of Alaska on Friday on orders from President Biden, White House officials said.

The White House National Security Council spokesman, John Kirby, said the object was downed because it was flying at about 40,000 feet and posed a “reasonable threat” to the safety of civilian flights, not because of any knowledge that it was engaged in surveillance. Asked about the object’s downing, Mr. Biden on Friday said only, “It was a success.”

Mr. Kirby described the object as roughly the size of a small car, much smaller than the suspected Chinese spy balloon downed by Air Force fighter jets Saturday off the coast of South Carolina after it transited over sensitive military sites across the continental U.S.

There were few answers about the object and the White House drew distinctions between the two episodes. Officials couldn’t say if the unknown object downed Friday contained any surveillance equipment, where it came from, or what purpose it had.

The Pentagon on Friday declined to provide a more precise description of the object, only saying that U.S. pilots who flew up to observe it determined it didn’t appear to be manned. Officials said the object was far smaller than the previous balloon, did not appear to be maneuverable, and was traveling at a much lower altitude.

Mr. Kirby maintained that Mr. Biden, based on the advice of the Pentagon, believed it posed enough of a concern to shoot it out of the sky — primarily because of the potential risk to civilian aircraft.

“We’re going to remain vigilant about our airspace,” Mr. Kirby said. “The president takes his obligations to protect our national security interests as paramount.”

The president was briefed on the presence of the object Thursday evening after two fighter jets surveilled it.

The Pentagon press secretary, Brigadier General Pat Ryder, told reporters Friday that an F-22 fighter aircraft based at Joint Base Elmendorf-Richardson shot down the object using the same type of missile used to take down the balloon nearly a week ago.

Ahead of the shoot-down, the Federal Aviation Administration restricted flights over a roughly 10-square-mile area within U.S. airspace off Alaska’s Bullen Point, the site of a disused U.S. Air Force radar station on the Beaufort Sea about 100 miles from the Canadian border.

The object fell onto frozen waters and officials expected they could recover debris faster than from last week’s downed balloon. General Ryder said the object was traveling northeast when it was shot down. He said several U.S. military helicopters have gone out to begin the recovery effort.

The unknown object was shot down in an area with harsh weather conditions and about six and a half hours of daylight at this time of year. Daytime temperatures Friday were about minus 17 degrees Fahrenheit.

The development came almost a week after the U.S. shot down a suspected Chinese spy balloon off the Carolina coast after it traversed sensitive military sites across North America. China insisted the flyover was an accident involving a civilian craft and threatened repercussions.

Mr. Biden issued the order but had wanted the balloon downed even earlier. He was advised that the best time for the operation would be when it was over water. Military officials determined that bringing it down over land from an altitude of 60,000 feet would pose an undue risk to people on the ground.

The balloon was part of a large surveillance program that China has been conducting for “several years,” the Pentagon has said.

China responded that it reserved the right to “take further actions” and criticized the U.S. for “an obvious overreaction and a serious violation of international practice.”

By Zeke Miller, Colleen Long, and Tara Copp