President Joe Biden ordered US warplanes to shoot down another mysterious object, this time near Lake Huron on Sunday, "out of abundance of caution," a senior administration official said.

The target of the fourth such intervention in just over a week -- described as an octagonal structure with strings hanging off it -- was not deemed to be a military threat to anything on the ground, but it could have posed a hazard to civil aviation as it flew at about 20,000 feet (6,000 meters) over Michigan, the official said.