Washington, Feb 13 (IANS): U.S. military officials said they are unsure how three unidentified flying objects shot out of the skies of North America had been able to stay aloft.

President Joe Biden ordered another object - the fourth in total this month - to be downed on Sunday, BBC reported.

As it was travelling at 20,000 ft (6,100m), it could have interfered with commercial air traffic, the U.S. said.

A military commander said it could be a "gaseous type of balloon" or "some type of a propulsion system."

He added he could not rule out that the objects were extra-terrestrials, BBC reported.

The latest object - shot down near the Canadian border - has been described by defence officials as an unmanned "octagonal structure" with strings attached to it, BBC reported.

It was downed by a missile fired from an F-16 fighter jet at 14:42 local time.

The incident raises further questions about the spate of high-altitude objects that have been shot down over North America this month.

U.S. Northern Command Commander General Glen VanHerck said that there was no indication of any threat, BBC reported.

"I'm not going to categorise them as balloons. We're calling them objects for a reason," he said.

"What we are seeing is very, very small objects that produce a very, very low radar cross-section," he added, BBC reported.

Speculation as to what the objects may be has intensified in recent days.

"I will let the intel community and the counterintelligence community figure that out," Gen VanHerck said when asked if it was possible the objects are aliens or extra-terrestrials, BBC reported.

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

A suspected Chinese spy balloon was downed off the coast of South Carolina on 4 February after hovering for days over the US. Officials said it originated in China and had been used to monitor sensitive sites.