Russia's FSB security service said on Monday it had arrested a senior Ukrainian intelligence officer for gathering secret information about Russian defense and security bodies.
Ties between Moscow and Kiev have been tense since 2014, when, after a popular uprising toppled Ukraine's pro-Russian president, Russia annexed the Crimea region from Ukraine and a pro-Russian separatist insurgency erupted in the country's east.
The FSB, successor to the Soviet-era KGB secret police, identified the arrested man as Roman Sushchenko and said he was an intelligence colonel in Ukraine's defense ministry.
"The Ukrainian citizen was focused on gathering information containing state secrets about Russia's armed forces and National Guard troops, the leak of which could have damaged the defense potential of the state," the FSB said in comments read on the state-controlled Rossiya 24 TV channel.
Rossiya showed a middle-aged man in handcuffs, saying the video footage had been taken by the FSB.
Asked to comment, a spokesman for Ukraine's military intelligence said: "We deny this information. He [Sushchenko] has nothing to do with us."
But Ukraine's state-run news agency Ukrinform said Sushchenko was its correspondent in France, who was in Moscow "on a private visit" on September 30 when he was detained. Sushchenko is a journalist with "a flawless reputation" and his unlawful arrest is "yet another brazen and illegal act by Russia against Ukrainian citizens," Ukrinform said.
Ukraine's Foreign Ministry said it was outraged by the arrest of Sushchenko, who it said had come to Russia to visit close relatives. It said the espionage charges brought against him were "trumped up".
The FSB said Sushchenko had been caught while "carrying out a spying action in Moscow". Russian news agencies said a Moscow court had charged Sushchenko with espionage and put him under pre-trial arrest for two months.
Russia's Foreign Ministry said Sushchenko had no journalist visa when he entered Russia.