A US civil rights advocacy group has launched a free mobile phone application that allows users to record police activity discreetly, saying it will help boost police force accountability.

The New Jersey branch of the American Civil Liberties Union (ACLU) explains that "Police Tape," available to the public since Tuesday, "allows people to securely and discreetly record and store interactions with police."

The Android phone application, available for download on the website of the ACLU of New Jersey, is "an essential tool for police accountability," said the office's executive director Deborah Jacobs.

"Too often, incidents of serious misconduct go unreported because citizens don't feel that they will be believed."

Unlike traditional smartphone recording apps, "Police Tape" disappears from the screen once it is launched, minimizing the possibility that police will notice the recorder.

The application allows users to send the file to the ACLU for safe-keeping and analysis.

"Police Tape" also contains legal advice on the rights of citizens confronted by police.

The ACLU of New Jersey website cites several court rulings that recognized citizens' rights to film police activity.

A similar application for iPhone will be available later this summer, the site says.