Washington: US Air Force launched a secretive unmanned "space plane" from Cape Canaveral, Florida on Saturday.

The X-37B Orbital Test Vehicle blasted off aboard an Atlas V rocket at 5.48 pm.

The Air Force said the second test flight would allow them to fine-tune the vehicle and try to replicate the initial results.

The secretive nature of the project has led to speculation about its role in the military - some say it can be used to spy on communications or to deploy small satellites. Some have expressed concerns it could mark the beginning of the weaponisation of space, a notion the Air Force flatly rejects.

The spacecraft weighs 4,990 kg and it is 8.9 metre long having a wingspan of 4.5 metre.

The vehicle is designed to remain in low orbit for up to 270 days. Built by Boeing's secretive Phantom Works division, the plane is powered by batteries and solar cells.

The X-37 programme began in 1999 under NASA's guidance before being transferred in 2004 to the Pentagon's Defence Advanced Research Projects Agency (DARPA), before winding up in the hands of the Air Force.