The FBI says it hasn't been able to retrieve data from nearly 7000 mobile phones in less than one year, as the US agency turns up the heat on the ongoing debate between tech companies and law enforcement officials.

FBI Director Christopher Wray says in the first 11 months of the fiscal year, US federal agents were blocked from accessing the content of 6900 mobile phones.

"To put it mildly, this is a huge, huge problem," Wray said in a speech on Sunday at the International Association of Chiefs of Police conference in Philadelphia.

"It impacts investigations across the board - narcotics, human trafficking, counterterrorism, counterintelligence, gangs, organised crime, child exploitation."

The FBI and other law enforcement officials have long complained about being unable to unlock and recover evidence from mobile phones and other devices seized from suspects even if they have a warrant. Tech firms maintain they must protect their customers' privacy.

In 2016 the debate was on show when the Justice Department tried to force Apple to unlock an encrypted mobile phone used by a gunman in a terrorist attack in San Bernardino, California. The department eventually relented after the FBI said it paid an unidentified vendor who provided a tool to unlock the phone and no longer needed Apple's assistance, avoiding a court showdown.

The Justice Department under US President Donald Trump has suggested it will be aggressive in seeking access to encrypted information from technology companies. But in a recent speech, Deputy Attorney General Rod Rosenstein stopped short of saying exactly what action it might take.