Poland has detained another member of a Russian spy ring, bringing the total number of people arrested in the investigation to 15, Interior Minister Mariusz Kaminski said on Monday, Reuters reported.

“The Internal Security Agency detained another member of the spy network who worked for the Russian special services,” Mariusz Kaminsky wrote on Twitter.

“The suspect was monitoring military facilities and seaports. He was systematically paid by the Russians,” he added.

The Russian Embassy in Warsaw did not immediately respond to a request for comment.

Poland, a key logistics hub for Western military supplies to Ukraine, says it has become a prime target for Russian spies and accuses Moscow of trying to destabilize the situation.

In June, Poland detained a Russian professional hockey player on espionage charges.

In March, Poland announced the liquidation of a Russian spy group and the detention of nine people who it said were preparing sabotage and monitoring railway routes to Ukraine.

In April, Poland announced it would introduce a 200-meter exclusion zone around its liquefied natural gas (LNG) terminal in Świnoujście, citing concerns about Russian espionage.