The objects shot down over North America over the past several days were unmanned and did not appear to send communication signals, the United States has said, as questions persist about the nature and the origin of the aircraft. White House national security spokesperson John Kirby told reporters on Monday that while Washington has no “specific reason” to suspect that the objects were conducting surveillance, it is not able to rule out the possibility.

“We assessed whether they posed any kinetic threat to people on the ground; they did not,” Kirby said. “We assessed whether they were sending any communication signals; we detected none,” he said. “We looked to see whether they were manoeuvering or had any propulsion capabilities; we saw no signs of that. “And we made sure to determine whether or not they were manned; they were not.”

Since Friday, three “high-altitude” objects have been detected in North American airspace and brought down in the US and Canada.