Norwegian police say they have warned politicians about possible eavesdropping of cellphone calls after several listening devices were reportedly found in central Oslo, including near government buildings and Parliament.

Siv Alsen from the security police says the National Security Authority has begun an investigation, but could not provide more information pending the agency's report.

Alsen told the AP that espionage "has been high" in Norway for several years and police have told politicians to be on their guard.

Her comments came on Sunday after media reports that illegal listening and tracking devices were found in fake mobile base stations, which could be used to monitor calls and data, as well as trace the movement of people in the area.

The National Security Authority did not immediately return calls.