The Ukrainian Security Service (SBU) says it has detained an alleged spy accused of collecting data on a "modern missile system" on behalf of Russia.
The SBU said in a statement that a resident of Ukraine's eastern region of Luhansk was detained on May 6 while trying to pass "secret technical documentation" regarding the Ukrainian system to Russia's Federal Security Service (FSB).
The detainee, who is accused of committing high treason, promised Ukrainian military personnel cash in return for the classified information.
Relations between Ukraine and Russia have been tense since Russia seized and annexed Ukraine's Black Sea peninsula of Crimea in March 2014.
Moscow has also fomented unrest and backed separatists in Ukraine’s eastern regions of Luhansk and Donetsk, where some 13,200 people have been killed in the ensuing conflict since April 2014.