Russian authorities have detained a Ukrainian citizen on suspicion of espionage in the city of Tula, some 200 kilometers south of Moscow.
The Federal Security Service (FSB) said on August 23 that the man, whose identity was not revealed, is suspected of collecting classified data related to Russia's latest firearms technologies for Ukraine's secret services.
If convicted, the suspect faces up to 20 years in prison.
There was no immediate reaction from Kyiv.
Earlier this year, Russian authorities in Ukraine's Crimea region arrested two men in separate cases in April and June on charges of spying for Kyiv.
Russia has arrested dozens of people in Crimea and across Russia on charges including espionage, extremism, and terrorism, since it seized Crimea in March 2014, sending in troops and staging a referendum denounced as illegitimate by at least 100 countries.
Rights groups have said that after imposing its control over the peninsula, Moscow aggressively moved to prosecute Ukrainian activists and anyone who questioned the annexation.
Russia also backs separatists in a war against Ukrainian government forces that has killed more than 13,200 people in eastern Ukraine since April 2014.