Slovakia has charged a former military academy officer and another man with spying for Russia, police and a special prosecutor said, Reuters reported.

The announcement came a day after the NATO country expelled three Russian diplomats. Russia said it would respond to the unjustified expulsion.

Acting police chief Stefan Hamran said at a press conference in Bratislava that the police had detained and charged a former vice-rector of the military academy named Pavol B. with cooperation with the Russian military intelligence GRU since 2013.

This person was an intelligence contact with Russian GRU officers for whom he was seeking, collecting and disclosing information of strategic importance, Hamran said.

Another suspect, he said, is Bohus G., an employee of a website that was shut down by the government earlier this month for spreading disinformation. According to Hamran, he admitted to seeking information from secret intelligence reports for Parliament.

Defendants can face from 4 to 13 years in prison if their guilt is proven. Two more people were detained but released without charge, police said.

Special Prosecutor Daniel Lipsich said the Ukraine crisis has brought renewed attention to espionage activities.

Russia has classified Slovakia as an unfriendly state, and for this reason, these cases will be given great attention, he said.