A Polish military court on Monday sentenced a high-ranking officer to six years behind bars after convicting him of spying for Russia.

The former lieutenant-colonel, identified only as Zbigniew J., had faced a 15-year maximum sentence at his trial behind closed doors at a military tribunal in Warsaw.

In the end, he received a relatively light sentence after cooperating with authorities, a spokesman for the court told the Polish PAP news agency.

The officer was arrested in October 2014 while working at the defence ministry's department of education and publicity.

A lawyer holding both Polish and Russian nationalities was also arrested in a simultaneous raid.

The lawyer has been charged with spying for Moscow but is still awaiting trial.

Prosecutors said the two cases were linked, but refused to confirm whether the men were suspected of having worked together.

Earlier this month, the leader of a small pro-Kremlin political party was also arrested on suspicion of spying.

In January, fellow EU and NATO member Estonia jailed three cigarette smugglers for spying for Russia's secret services.

That incident followed a Cold War-style spy swap between Russia and Estonia in September on a bridge spanning their shared border.

Poland, a former Eastern Bloc member, and Estonia, a former Soviet republic, are among the most vocal of NATO members demanding the alliance beef up its eastern defences in the wake of the Ukraine crisis.

Russia's annexation of the Crimea peninsula from Ukraine in 2014, and the pro-Moscow revolt in eastern Ukraine that followed, prompted the worst stand-off between Moscow and the West since the Cold War.