The company whose helicopter crashed and killed basketball star Kobe Bryant, his daughter and seven others was not certified to fly in foggy conditions where pilots relied on only cockpit instruments, US officials say.

Island Express Helicopters is limited to operating under visual flight rules, meaning pilots must be able to see clearly outside the aircraft in daylight, National Transportation Safety Board spokesman Keith Holloway said.

“The preliminary information is Island Express’s 135 certificate did not allow for IFR flight,” Mr Holloway said on Friday. “No other specifics are available at this time.”

The aircraft was however equipped for instrument flying, multiple media reports said.

“There is only one way you can be in the clouds, on an IFR flight plan or by accident,” pilot and former Island Express safety manager Kurt Deetz told The New York Times, referring to instrument flight rules.

Mr Holloway told Reuters on Friday it was unknown if the pilot was in fact flying on instruments at the time of the wreck.

He said a preliminary report on the crash, expected in about 10 days, may include such a determination.

The helicopter’s pilot Ara Zobayan was licensed for instrument flying, but likely had little experience in doing so given the company’s operating limitations, Mr Deetz told Forbes separately.

The twin-engine helicopter slammed into a hillside in Calabasas, California, with clouds and fog limiting visibility.

Air traffic controllers had given Mr Zobayan “special visual flight rules”, or clearance to fly in the less-than-optimal weather around the Burbank airport.

The pilot had reported that conditions were sufficient for visual flight, the Times said, adding that the weather appeared to have worsened as the flight continued.

In a separate statement, Island Express said it was suspending all services.

“The shock of the accident affected all staff, and management decided that service would be suspended until such time as it was deemed appropriate for staff and customers,” the charter company said.

The death of Bryant, 41, an 18-time NBA all-star and one of the most admired athletes around the globe, sent shock waves through the sports and entertainment worlds.

-AAP