LOS ANGELES (AFP) - A rocket science safety expert on Sunday said Virgin Galactic "ignored" safety warnings in the years leading up to the deadly crash of its spacecraft in California, as investigators hunted for clues to accident's cause.

Carolynne Campbell, a rocket propulsion expert with the Netherlands-based International Association for the Advancement of Space Safety, said she could not speculate on the cause of Friday's crash without "all the data".

However, she said multiple warnings had been issued to Virgin since 2007, when three engineers died testing a rocket on the ground.

"Based on the work we've done, including me writing a paper on the handling of nitrous oxide, we were concerned about what was going on at Virgin Galactic," she said.

"I sent copies of the paper to various people at Virgin Galactic in 2009, and they were ignored," she said.

Campbell said she outlined concerns to Virgin Galactic in a subsequent telephone conversation, but her warning again went unheeded.

"I warned them... that the rocket motor was potentially dangerous," she said.

Campbell's warnings related to nitrous oxide, which has been used as a fuel component in previous test flights.

Friday's flight was reportedly the first using a new kind of fuel derived from nylon plastic fuel grains, but it was not clear whether nitrous oxide was also involved.

Tom Bower, who wrote an unauthorised biography of Branson, told BBC radio on Saturday several Virgin Galactic engineers had left the company in recent years over safety concerns in the wake of the deadly 2007 accident.

"All the engineers in California working on the project I've spoken to said it was very dangerous," Bower said.

A team of investigators from the National Transportation Safety Board (NTSB) has been deployed to the Mojave Desert to probe Friday's accident, which left pilot Michael Alsbury dead and and co-pilot Pete Siebold seriously injured.

British tycoon Richard Branson told reporters in Mojave on Saturday safety had always been Virgin's paramount concern.

"Safety has always been our number one priority," Branson said, adding that the company would not "push on blindly" with its ambitious space program until the causes of the accident had been determined.

Branson, however, took aim at early speculation of the causes of the crash, which had focused on the new rocket fuel.

"To be honest, I find it slightly irresponsible that people who know nothing about what they're saying can be saying things before the NTSB makes their comments," he said.

Witnesses to Friday's crash say there was no obvious sign of an explosion before Virgin's suborbital SpaceShipTwo broke apart and hurtled to earth shortly after it had detached from a mothership at an altitude of around 45,000 feet (13,700 meters).

The crash was the second disaster to rock the private space industry in the space of a few days, after an Antares rocket carrying supplies to the International Space Station exploded after take-off in Virginia in Tuesday.

Experts say the accident will delay the advent of commercial space tourism by several years.