A mysterious US Air Force spacecraft caused a sonic boom when it landed at NASA's Kennedy Space Centre in Florida.

The unmanned X-37B aircraft landed successfully on Sunday morning, after nearly two years in orbit.

There has been no official clarification of what the reusable plane does during its long missions.

Lieutenant Colonel Ron Fehlen, X-37B program manager, said only:

"The landing of OTV-4 marks another success for the X-37B program and the nation.

"This mission once again set an on-orbit endurance record and marks the vehicle's first landing in the state of Florida. We are incredibly pleased with the performance. 

"We are extremely proud of the dedication and hard work by the entire team."

There's been some speculation that the aircraft could be testing a new kind of satellite weapon or conducting surveillance. 

The US military is planning to launch the fifth X-37B mission later this year from Cape Canaveral Air Force Station.

Image: File photo of an X-37B.