The Paris mayor's office denied Wednesday that Olympics security plans had been stolen from one of its employees.

A police source speaking to AFP and the BFM news channel had said Tuesday that a USB key containing plans for the Paris municipal police during the Olympics had been lost when a city hall employee had his bag stolen on a train.

The employee put the bag on a luggage rack above his seat and discovered the loss at the busy Gare du Nord station on Monday evening, the reports said.

In a statement on Wednesday, the city said that "initial investigations have established that the employee was not in possession of any information about the organisation or the deployment of security forces during the Olympics and Paralympics."

It confirmed that a police complaint over a theft had been made, but said the lost items contained "personal notes related to his work in IT support for the department of roads and transport."

The Paris prosecutors' office also said that the USB key "contained only notes about road transport in Paris during the Olympics, and not sensitive security information."

Around 35,000 security forces are expected to be on duty each day for the Olympics from July 26-August 11, including around two thousand municipal police officers from the capital.

As well as managing crowds and preventing street crime, police will be on high alert for possible terror plots, with France a frequent target of Islamist extremists over the last decade.

The city employee is set to be investigated to see if he broke any internal security rules by travelling with the USB key, the statement from Paris city hall said.

The Paralympics are set to take place from August 28-September 8.