One pilot has died and another is seriously injured after a rocket belonging to billionaire Sir Richard Branson's space tourism venture crashed during a test flight, US authorities said.
Virgin Galactic's SpaceShipTwo rocket came down in the Mojave Desert in California.
An eyewitness reported that the craft exploded in flight after ignition.
A Virgin Galactic spokesman said the rocket "suffered a serious anomaly" after a test flight conducted by partner business Scaled Composites.
California Highway Patrol said one person was dead and another had suffered a major injury. The rocket is usually flown by two pilots.
Sir Richard wrote on Twitter: "Thoughts with all @virgingalactic & Scaled, thanks for all your messages of support. I'm flying to Mojave immediately to be with the team."
The identities of the two pilots have not yet been released. Virgin Galactic's chief pilot is Scots-born Dave Mackay, who grew up in Sutherland. 
SpaceShipTwo has been under development at Mojave Air and Spaceport.
It was designed to be carried into the air by the WhiteKnightTwo jet and then released before igniting its rocket to travel into space, before returning to Earth as a glider.
A Virgin Galactic spokesman said: "Virgin Galactic's partner Scaled Composites conducted a powered test flight of SpaceShipTwo earlier today.
"During the test, the vehicle suffered a serious anomaly resulting in the loss of SpaceShipTwo. WhiteKnightTwo landed safely.
"Our first concern is the status of the pilots."
He added: "We will work closely with relevant authorities to determine the cause of this accident and provide updates as soon as possible."
Photographer Ken Brown said he saw SpaceShipTwo get released from WhiteKnightTwo but explode after its rocket motor was ignited.
A spokeswoman for America's Federal Aviation Administration said: "Just after 10am PDT (5pm GMT) today, ground controllers at the Mojave Spaceport lost contact with SpaceShipTwo, an experimental space flight vehicle.
"The incident occurred over the Mojave Desert shortly after the space flight vehicle separated from WhiteKnightTwo, the vehicle that carried it aloft. Two crew members were on board SpaceShipTwo at the time of the incident. WhiteKnightTwo remained airborne after the incident. The FAA is investigating."
Former Nasa astronaut Michael Massimino said the crash brought back memories of the loss of Space Shuttle Columbia in 2003, which disintegrated during its re-entry into Earth's atmosphere, killing all seven crew members on board.
Mr Massimino told CNN: "It reminds me of when we had our accident in 2003, when we lost Columbia."
He added: "It was a really bad day, bad for everyone involved, particularly the families. It's a tragedy, it's a nightmare. When we lost Columbia it was the worst day of my life."