The pilots of US Air Force F-35 fighters sent to intercept the mystery object over Alaska on Friday say that whatever it was had been "interfering with the sensors of their aircraft" before they shot it down.

Three unidentified objects have been spotted over North America in the past 10 days. The first, a giant airship "the size of three buses" is understood to have originated in China although the US and Chinese governments disagree about its exact purpose.

While the Pentagon says it was a spy balloon, China says the monster airship was a civilian craft designed to monitor weather systems – but even if that turns out to be correct, the two smaller objects sighted over the US in recent days remain unexplained.

The USAF pilots who shot down the object over Alaska "were not able to identify what they saw," reports CNN's Natasha Bertrand.

"When the US first detected this object over Alaska on Thursday, they sent up F-35 jets to … see what was going on," she explained. "And these pilots reported back very conflicting accounts".

The pilots of the cutting-edge stealth fighters said "that this object was actually interfering with the sensors of their aircraft and they couldn't figure out why, because there was no identifiable kind of surveillance equipment on the object," she said.

The pilots were also mystified as to what kind of aircraft they were actually dealing with.