The US military has landed its robotic space plane, ending a classified 22-month mission that marked the third in Earth orbit for the experimental programme widely believed to be related to spying.

The X-37B touched down at Vandenberg air force base in California on Friday, bringing to a close the third and longest mission the vehicle has undertaken since its maiden voyage in 2010.

The spacecraft conducted unspecified experiments for 674 days while in orbit. The US air force said the orbiter, built by Boeing, performed "risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies", although details of the missions are secret.

In a written release announcing the craft's return, the air force only said it had been conducting "on-orbit experiments".

"I'm extremely proud of our team for coming together to execute this third safe and successful landing, said commander Keith Balts of the US military's 30th space wing.

The Secure World Foundation, a nonprofit group promoting the peaceful exploration of space, says secrecy surrounding the orbital activities and payloads of the X-37B are almost certainly due to the presence of spy hardware being tested.

US officials have previously denied the project had anything to do with creating a "space weapon" that could knock down other satellites.

The plane stands 2.75m tall and is nearly 9m long, with a wingspan under 1.4m. It weighs 5,000kg and has solar panels that unfurl to charge its batteries once in orbit. It travels at up to 25 times the speed of sound as it circles the earth.

The US air force is preparing to launch the fourth X-37B mission from Cape Canaveral in 2015. The expected length of its next mission is unknown.