A young boy had an unexpected encounter when he took video footage of the sky outside the aircraft he was travelling in.

The youngster was flying to Massachusetts in the United States when he decided to film another passenger jet flying in the distance.

But soon after he pressed 'Record', a light, flat object sped past at very high speed. The voice of the boy is heard on tape saying: "Whoa! Did you see that?"

Not even slowing down the footage by 50 percent could unravel the identify of the mystery object.

Experts were divided on whether the image was merely condensation moving along the outside of the window pane or something more otherworldly.

The footage was posted on YouTube and picked up by RealUFOs.net, which hosts footage shot by spotters who do not wish to be identified.

"This footage was shot while on a flight to Massachusetts in January 2013," reads the description.