Cab firm Uber has been accused of not doing enough to check the backgrounds of its drivers in the US.

Former murderers, sexual offenders and thieves have all been Uber drivers, allege court papers filed by district attorneys in California.

They started legal action against Uber last year, saying it had misled customers about its screening systems.

Uber said its driver checking system was "more effective" than those used by other taxi firms.

The court papers claim that Uber did not spot the criminal backgrounds of 25 drivers that it employed in San Francisco and Los Angeles - the two cities behind the legal action.

At a press conference, George Gascon, district attorney in San Francisco, said problems with the data that Uber relied on to check drivers meant it could miss some former criminals. For instance, he said, 30,000 registered sex offenders were not in the database Uber used.

An alternative screening system used by other cab firms called Livescan did catch people who were on the sex offenders list, said Mr Gascon.

He added that Uber's screening only covered the last seven years and omitted biometric checks that would uncover people who had falsified their identity to avoid being spotted by screening.

"We are learning increasingly that a lot of the information that Uber has been presenting the consumer has been false and misleading," Mr Gascon told the New York Times.

In response, Uber said it agreed with the two district attorneys that safety was a priority - but took issue with whether Livescan was a better way to check backgrounds.

Neither process was 100% foolproof, it said in a statement, adding that it had put "hundreds" of people registered as taxi drivers through its own checks and found that many who were driving cabs, but were not Uber drivers, had convictions for rape, attempted murder, child abuse and violence.

"We look forward to resolving this issue, just as the DAs settled an almost identical case with [rival cab firm] Lyft last year," it said.

In the UK, Uber said its drivers were subject to the same strict checks that covered those operating black cabs or other licensed vehicles.