US President Joe Biden ordered the military to down the object. Photo / AP

The Pentagon has shot down an unknown object flying in United States airspace off the coast of Alaska this morning, White House officials said.

The object was flying at about 40,000 feet (12.1km) and posed a “reasonable threat” to the safety of civilian flights, said White House National Security Council spokesman John Kirby,

Kirby said US President Joe Biden ordered the military to down the object, which he described as roughly the size of a small car.

It’s the second time in a week US officials have downed some type of flying object over the US.

Last week, fighter jets shot down a suspected Chinese spy balloon off the coast of South Carolina.

White House officials drew major differences between the two episodes. Kirby said it wasn’t yet known who owned the object, and he did not say that it was a balloon.

Officials also couldn’t say if there was any surveillance equipment on it. He also didn’t know yet where it came from or what its purpose was.

Kirby said fighter pilots visually examining the object ascertained that it was not manned.

The object fell into frigid waters and officials expected they could recover debris faster than from last week’s massive balloon.

The development came almost a week after the US 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.

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 (18.2km) would pose an undue risk to people on the ground.

China responded that it reserved the right to “take further actions” and criticised the US for “an obvious over-reaction and a serious violation of international practice.”

- AP