The United States said Monday it does not know the origin of the three high-altitude objects it shot down over the past few days as they drifted in the winds over North America.

The government said it does not believe the objects were surveillance aircraft, though it is leaving open the possibility that they may be.

"They didn't have propulsion," National Security Council spokesperson John Kirby told reporters at the White House. "They were not being maneuvered. They didn't have surveillance [capability], but we couldn't rule it out."

"We're sort of in uncharted territory here," Kirby said.

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