"The ABW counter-intelligence agency has arrested nine people suspected of working for the Russian secret service," Interior Minister Mariusz Kaminski told reporters.

"The suspects had been conducting espionage activities against Poland and preparing acts of sabotage on behalf of Russian intelligence services," he added.

Kaminski said they were "foreigners from across Poland's eastern border".

Polish Defence Minister Mariusz Blaszczak had earlier said, "The whole network has been dismantled," adding, "the threat was real".

Six of the suspects have been provisionally charged with espionage and participation in an organised criminal group. The other three were still being questioned.

Kaminski said the group's tasks had been "reconnaissance, surveillance and documentation of arms transports to Ukraine".

"The suspects had also been preparing acts of sabotage meant to paralyse the delivery of military equipment, arms, and Ukraine aid," he added.

ABW agents seized electronic equipment, as well as GPS transmitters due to be installed on trains carrying aid to Ukraine.

On Wednesday, private Polish radio station RMF, citing unnamed sources, was first to report on the alleged spy ring operation.

It said the suspects were arrested after the discovery of hidden cameras on important railway routes and junctions, recording and transmitting data on traffic.

According to RMF, "dozens of devices" of this type were installed, mainly on sections of railways leading to the southeast, including near an airport that is one of the main transfer points for Ukraine-bound Western weapons and ammunition.

Authorities are now on high alert and the security of railroads and strategic infrastructure has been reinforced, according to RMF.

Kaminski also said the group had been tasked with carrying out propaganda activity to destabilise Polish-Ukrainian relations as well as fomenting anti-NATO sentiment in Poland.

He said Poland has evidence that the group's members received regular payment from Russia's secret services.