London, Oct 18 : The US Air Force's X-37B space aircraft has recently returned to Earth after a two year of secret mission, it has been reported.

The plane, which spent 674 days in orbit around the planet, touched down at Vandenberg Air Force Base in California, the BBC reported.

The real purpose of the plane remains unclear; a theory that it was taking a look at China 's space lab has recently been downplayed by experts.

However, Air Force officials told US media that the aircraft performs "risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies."

It was the third mission for the programme, started in 1999 and currently run by the Air Force's Rapid Capabilities Office.

A fourth X-37B mission has been said to currently be slated for launch from Cape Canaveral, Florida, in 2015.(ANI)