"There was an explosion on the Mojave Airport at 2:30 this afternoon (2130 GMT)," fire engineer Roberto Figueroa of the Kern County Fire Department told AFP. "There are two confirmed fatalities and four seriously injured."

"What exploded was a rocket," he said.

The cause of the explosion was not immediately known.

The airport is located 130 kilometers (80 miles) north of Los Angeles.

Local television CBS2 broadcast images taken from a helicopter showing debris scattered over a wide area and semi-trailer split by the blast.

According to the station, the explosion erupted from tanks filled with rocket fuel during engine tests carried out at the airport.

The Mojave desert site near the US Edwards Air Force Base is home to several private space flight companies.

Among them is Scaled Composites, an aerospace company run by US engineer Burt Rutan, who in 2004 won a 10-million-dollar prize by launching a rocket to space twice within a two-week period.

Scaled Composites, which is currently working with British billionaire Richard Branson's Virgin Galactic to send tourists to space by 2010, did not answer AFP telephone calls late Thursday.

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