A classified American spy plane is "probably" spying on China, a report in Spaceflight magazine has claimed.

Neither the Pentagon or the US Air Force have revealed what mission their X-37B vehicle, which is unmanned and has been in space since March, is undertaking.

However amateur space researchers have been tracking the plane, and say they have compiled enough evidence to reveal what it's up to from a low orbit about 300km above the Earth.

According to Spaceflight and the BBC, the X-37B's path is almost exactly the same as China's Tiangong-1 space laboratory.

"We think the X-37B may be using to maintain a close watch on China's nascent space station," Spaceflight editor Dr David Baker told the BBC.

Other researchers quoted in the Spaceflight article, which will be published on Saturday, claim that if the Pentagon wanted to spy on Tiangong-1 it could do so without using the X-37B, and argue the similarity in their flight path could just be a coincidence.

The US Navy has kept tight-lipped about the X-37B, saying merely that it is a testbed for new technologies.

The Boeing-built craft, which is about 9m long, reusable and can automatically land on a runway. It also has space for a payload about the size of a small van - but what is carried inside is not known.

China expects to send its astronauts to the lab later this year. Whether or not they'll be greeted by the mysterious American spaceship or not is yet to be seen.

Of course given the state of the Pentagon's research into creating time holes to hide entire events inside, it's possible they'll never know either way.