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Ride-sharing firm Uber is facing a criminal investigation by the US government.

The scrutiny has started because the firm is accused of using "secret" software that let it operate in regions where it was banned or restricted.

The software, called "greyball", helped it identify officials seeking to stop the service running.

A spokesman for Uber declined to comment on the investigation, .

It is claimed greyball was used in several areas, including Portland, Oregon, where the ride service was still seeking official approval to operate.

In those areas, transport regulation officials posed as passengers in a bid to prove that the company was operating illegally. Greyball worked out who the officials were and blocked them from booking rides with the company's drivers.

In a letter sent last week to transport regulators in Portland, Uber said it used the greyball software "exceedingly sparingly" in the city and had not used it since April 2015 when it was granted permission to operate.

Uber's use of the software was revealed by the New York Times earlier this year. Uber defended its use in a blog saying the software helped it work out if a ride request was legitimate. It helped Uber limit fraud and protect drivers from harm, it added.

It is not clear what sanctions Uber will face if the investigation finds that it did act illegally.

The criminal inquiry comes at a difficult time for Uber which has faced criticism on many fronts. It is currently fighting a lawsuit from Google-backed self-driving car firm Waymo.