Russia's domestic security agency says it has captured a Ukrainian security officer who volunteered to spy for Moscow and will send him back because they believe he is a double agent.

The Federal Security Service, or FSB, the main KGB successor agency, said in a statement carried by Russian news agencies that the man, Yuri Ivanchenko, was detained on Saturday.

The agency said Thursday that Ivanchenko traveled to Moscow to offer his services to the FSB. It claimed that the CIA had helped the Ukrainian security service prepare Ivanchenko for the mission aimed at eventually exposing his Russian contacts. It said Ivanchenko will be sent home.

Russia and Ukraine are locked in a tug-of-war after Moscow's annexation of Ukraine's Crimean Peninsula and support for a separatist insurgency in eastern Ukraine.