Two Ukrainian nationals have been detained in Poland on suspicion of spying for a foreign intelligence service, authorities in Warsaw said Monday morning.

Prosecutors handling the case allege that the pair, aged 32 and 34, gathered classified data on “soldiers of the Polish Armed Forces, critical infrastructure located on the territory of the Republic of Poland, including transport infrastructure providing logistical and military support to Ukraine,” the Polish counterintelligence agency ABW said in a statement.

The ABW also said the suspects had installed monitoring devices near key facilities to enable “covert tracking of critical infrastructure.” They received payments for their work, the ABW also said.

Poland has been on high alert for cases of foreign espionage and sabotage both on the ground and in cyberspace, which authorities have linked directly to Russia or its close ally Belarus. 

The arrests, which took place on Oct. 14 in the southern Polish city of Katowice, are part of  an espionage probe overseen by prosecutors and based on information from the Military Counterintelligence Service SKW. 

The two Ukrainian nationals were charged with readiness to work for a foreign intelligence service and collecting sensitive information, a crime subject to imprisonment from six months to eight years. 

Other recent incidents in Poland involved an alleged Belarusian refugee, who Poland says was an operative for Russia, setting fire to a shopping mall near Warsaw; and an alleged attempt that is being investigated to sabotage a railway station by leaving an unmarked railcar on tracks used by passenger trains.