Ukrainian state security agents searched the offices of Russian state news agency RIA Novosti and detained the director of its Kiev office on Tuesday, accusing RIA of being used in an "information war" by Russia against Ukraine.
Relations between Kiev and Moscow nose-dived after Moscow annexed Crimea from Ukraine in 2014 and backed a separatist insurgency in Ukraine's eastern Donbass region that has killed more than 10,000 people.
Ukraine says Russia is fighting a "hybrid war" with Kiev, spreading propaganda while supporting the separatists with troops and sophisticated weaponry and launching cyber attacks on Ukrainian infrastructure, which Moscow denies.
The Kiev authorities have previously detained or deported several journalists for spreading anti-Ukrainian propaganda and supporting the separatist cause.
"Law enforcers established that they were used by the aggressor country in the context of a hybrid information war against Ukraine," the state security service (SBU) said in a statement.
RIA Novosti Ukraine confirmed the search of its premises and the detention of journalist Kirill Vyshinsky.
"We have a sound evidence base, which will be made public after the completion of the investigation," SBU press secretary Olena Gitlyanska wrote on Facebook.
The Kremlin condemned Kiev's actions and promised to protect the interests of Russian media.
"For now, we only know that there are forceful actions against the Russian media. This is enough for tough condemnation," Kremlin spokesman Dmitry Peskov said.
"We will act aggressively and protect the interests of Russian media to the fullest extent."