The United States does not know if unidentified objects shot down by American warplanes in recent days were spying, the White House said Monday.
"We don't know for sure whether they had a surveillance aspect to them, but we can't rule it out," White House national security spokesman John Kirby said in a press conference.

The White House also said there is no sign of alien activity related to the unidentified objects, after a top US officer declined to rule it out.

"There is no indication of aliens... with these recent takedowns, (I) wanted to make sure that the American people knew that," Press Secretary Karine Jean-Pierre told journalists.

The day before, General Glen VanHerck -- the head of the US Northern Command -- was asked during a briefing if aliens or extraterrestrials had been ruled out.

"I haven't ruled out anything. At this point, we continue to assess every threat or potential threats unknown that approaches North America with an attempt to identify it," VanHerck said.

A US warplane shot down an alleged Chinese surveillance balloon on February 4 and three unidentified objects were downed in subsequent days.

It is not clear whether the three unidentified objects -- shot down Friday over Alaska, Saturday over Canada and Sunday over Lake Huron -- have any connection to a much bigger white balloon that was downed over the Atlantic.