Russia's Supreme Court has upheld a 14-year prison sentence for a Polish citizen convicted of spying, the Federal Security Service (FSB) said Monday.

A Moscow court in June found Marian Radzajewski guilty of trying to obtain military information that included Russian "state secrets" after a closed trial.

Russia's FSB security service said the Supreme Court on Thursday ruled the sentence was "legal and justified." 

The FSB has said that the Polish citizen tried to obtain "secret components of Russian S-300 air defense systems." 

It said Radzajewski was arrested as he was attempting to complete the transaction.

The FSB has accused Radzajewski of acting "in the interests of a Polish organization that is a leading supplier of [Poland's] national armed forces and special forces." 

Radzajewski was reportedly born in 1977 and was the director of a transport company in Bialystok, a city in northeastern Poland. 

According to an investigation into his case by respected Polish magazine Polityka, Radzajewski's transport company could have been involved in bringing components from Russia and Belarus for the Polish army that still uses some Soviet-made equipment. 

Spy convictions have increased in Russia in recent years. Many of them are related to Ukraine, but cases concerning Poland and the Baltic states are not uncommon.