An unidentified aircraft was seen flying among other airliners in skies above Oregon - causing confusion among air traffic control and the Air Force, which sent F-15s to investigate.

The aircraft flew over Oregon on October 25, with no flight plan, no active identification transponder or transmitting collision avoidance signals.

While information about plane's pilot or intended destination remain unclear, some have suggested that the aircraft was trafficking drugs.

Additionally, it is believed that the mysterious flight could be a potential breach of national security.

An unidentified aircraft flew over Oregon on October 25, with no flight plan, no active identification transponder or transmitting collision avoidance signals. The Air Force had to send F-15s to investigate, but they couldn't find the aircraft. Pictured, a file photo of an F-15

FAA and North American Aerospace Command (NORAD) confirmed that the aircraft had flown over Oregon, the War Zone blog of The Drive reported.

The aircraft first caused alarm when the Seattle Center struggled to make contact with it. Meanwhile, air traffic control stations were reportedly having difficulty following it on radar.

The FAA asked NORAD to help track the aircraft at heights of 35,000 and 40,000ft - where commercial flights usually fly - but failed to find anything.

Eventually the US Air Force's F-15C interceptor fighters were launched to investigate but never found the aircraft, despite having advanced combat search and tracking radar technology aboard.

It's believed that the aircraft was flying at a speed equal or higher to that of a commercial airliner.

It still remains unclear who was flying the aircraft or where it was headed.

Oregon is north of Nevada's Groom Lake, the US Air Force testing facility better known as Area 51.

Reddit user Duprass said he was flying from the Bay Area to Seattle when he heard about the unidentified aircraft.

'Just landed in Seattle coming from the bay area,' Duprass said about the incident. 'Beginning over Southern Oregon we kept overhearing Seattle Center attempting to track an airplane with no transponder who wasn't talking.

'A handful of crews were able to track it visually, best they could tell it was between FL350-370 [35,000-37,000 feet], northbound. Nobody close enough to see the type.

Last we heard it was over the Willamette valley northbound and some fighters, perhaps out of PDX [Portland International Airport], were scrambled to go take a peek. Center had trouble tracking it on primary radar.'

The user updated the post by saying: 'Called SEA ARTCC. The gentleman I spoke with said that they initially got alerted to the aircraft from Oakland Center who was painting it on primary [illuminating it with radar, but without transponder information].

'For whatever reason they couldn't track it themselves on primary, and that's when I overheard them using airline aircraft to track it visually.

'The last airplane to see it had to descend into Portland and lost sight of it. The fighters were scrambled out of PDX but flew around for a while and did not find it. And that's that.'