Ukrainian authorities say a Ukrainian citizen with permanent residence in Russia has been detained in the northwestern city of Rivne.
Ukraine's Security Service (SBU) said on October 9 that the man, whose identity was not disclosed, is suspected of attempting to obtain classified information by offering a large sum of money and Russian citizenship to a Ukrainian military officer.
The suspect was detained on October 8, one day after Russian authorities officially charged Ukrainian journalist Roman Sushchenko, who was detained in Russia on September 30, with espionage.
Ukraine-Russia ties have deteriorated since Moscow forcibly annexed Ukraine's Crimean Peninsula in 2014 and supported pro-Russia separatists fighting in eastern Ukraine.
Meanwhile, Ukrainian Foreign Minister Pavlo Klimkin said on October 9 that Kyiv is considering leaving the Commonwealth of Independent States, of which Russia and eight other former Soviet republics are members.
Ukraine and Turkmenistan are only associate members of the bloc.