A senior U.S. senator believes the fingerprint recognition technology featured in Apple's new iPhone 5S raises "substantial privacy questions", BBC News reports.

Senator Al Franken, chairman of the influential Senate Judiciary Subcommittee on Privacy, Technology and the Law, has written to Apple boss Tim Cook explaining his security concerns.

After stealing someone's thumbprint, hackers could "impersonate you for the rest of your life," he wrote.

Apple has yet to comment on the letter.

Mr Franken wants answers to a number of questions, such as: whether the fingerprint data stored locally on the mobile phone chip in encrypted form could ever be stolen and converted into digital or visual form that would be usable by hackers or fraudsters; whether the iPhone 5S transmits any diagnostic information about the Touch ID system back to Apple or any third parties; how well customer fingerprint data will be protected and kept private; the exact legal status of such fingerprint data.

Mr Franken has asked Apple to answers his questions within a month of receiving his letter.

Meanwhile, hackers are gearing up to try to crack Apple's Touch ID technology.

The website istouchidhackedyet.com, set up by Nick DePetrillo and Robert Graham, lists a number of people offering rewards - including one for $10,000 from IO Capital, a venture capital company - "to the first person who can reliably and repeatedly break into an iPhone 5S by lifting prints (like from a beer mug)."