He added he could not rule out that the objects were extra-terrestrials.
The latest object - shot down over Lake Huron in Michigan near the Canadian border - has been described by defence officials as an unmanned "octagonal structure" with strings attached to it. บาคาร่า99
It was downed by a missile fired from an F-16 fighter jet at 14:42 local time (19:42 GMT).
The incident raises further questions about the spate of high-altitude objects that have been shot down over North America this month.
US Northern Command Commander General Glen VanHerck said that there was no indication of any threat.