An explosion on Thursday killed two workers and critically injured four others at a Mojave Desert airport site used by the pioneering aerospace company that sent the first private manned rocket into space, authorities said.

The blast at a Mojave Air and Space Port facility belonging to Scaled Composites LLC released nitrous oxide, or laughing gas, into the air. Hazardous material teams were on the scene as a precaution and fire authorities said the scene was safe. All the victims worked for Scaled, the Mojave-based builder of SpaceShipOne, the first private manned rocket to reach space.

Aerospace designer Burt Rutan, who heads Scaled but was away, rushed back to Mojave. He appeared emotional, hugging the airport manager and fire chief. His voice trailed off at times as he spoke to reporters. No information about the victims was released because families were being notified.

Rutan said the blast did not involve a rocket firing but happened during a test of the flow of nitrous oxide through an injector in the course of testing components for a new rocket motor for the upcoming SpaceShipTwo.

The nitrous oxide was at room temperature and under pressure, Rutan said.

Rutan gave little additional information about the test, but said it had been done safely many times during the SpaceShipOne program and had been done once before for the SpaceShipTwo program.

"We were doing a test we believe was safe. We don't know why it exploded. We just don't know," he said.

SpaceShipTwo is to be used for the new space tourism business Virgin Galactic belonging to Richard Branson. The company plans to offer $200,000 rides into space for tourists.

Scaled's offices and aircraft construction facilities were closed late Thursday. Authorities did not allow access to the blast site in a remote unpaved area about a quarter-mile beyond an airplane storage area.

Video news helicopters showed wrecked equipment and vehicles at the airport in the high desert north of Los Angeles near Edwards Air Force Base.

Scaled uses nitrous oxide as an oxidizer in its rockets, which are tested at the airport. An oxidizer provides the oxygen that rocket fuel needs to burn. Scaled's website notes that "temperatures and pressures must be carefully controlled" during oxidizer transfers.

Paramedics reported two people were killed, four were critically injured and one suffered minor injuries, said Mark Corum, a spokesman for Hall Ambulance Service. The injured were airlifted to Kern Medical Centre about 70 kilometres from the airport, he said.

The Mojave airport is where the Rutan-designed Voyager aircraft was built. It made history in 1986 when it achieved the first non-stop flight around the world without refuelling.

In 2004, Rutan's SpaceShipOne, funded by Microsoft co-founder Paul Allen, made the first privately financed manned spaceflight by climbing more than 100 kilometres high on a suborbital journey above Mojave. SpaceShipOne went on to make two more flights to win the $10 million USAnsari X Prize.

Rutan has since been developing SpaceShipTwo for Branson, who is investing at least $200 million US for a fleet of Rutan's spaceships. Earlier this year he told a trade show the new ship will be ready within a year and, after a year of flight tests, would have its first commercial launch in 2009.