Polish authorities said they have arrested a man suspected of spying for Russia, the Polish internal security agency said in a statement Monday.
The 43-year-old is accused of “providing the Russian special services with information and materials in the military field and about Polish entities and citizens.”
The ISA said its officers detained him on May 5.
The unnamed suspect pleaded guilty to the charges and was placed in pre-trial detention for three months, the ISA said.
It added that “much of the investigative material is classified.”
The Russian state-run TASS news agency identified the suspect, a Polish citizen, by his first name Marcin and the last initial K.
Ties between Russia and EU member Poland have deteriorated over Moscow’s annexation of Crimea from Ukraine and the conflict between pro-Russian separatists and government forces in eastern Ukraine in 2014.
The two countries have also been at odds over the role of the Soviet Union in World War II.
Last month Warsaw expelled three Russian diplomats over what it called "hostile actions" by Moscow following similar expulsions by the United States.