Ukraine's security service raided the Kiev offices of Russia's state-run RIA Novosti news agency on Tuesday, detaining one journalist who was accused of treason.
The searches took place with relations between Moscow and Kiev already extremely strained over Russia's 2014 seizure of Crimea and accusations that it fuelled a rebel conflict in eastern Ukraine that has cost some 10,000 lives.
The raids and arrest came on the day that Russian President Vladimir Putin opened a bridge providing the first direct road connection from southern Russia to the Crimean peninsula.
Olena Gitlyanska, a spokeswoman for the Security Service of Ukraine (SBU), said Russian-controlled media were being "used as tools in a hybrid war against Ukraine".
Earlier RIA Novosti journalist Kyrylo Vyshynsky, a Ukrainian national who received a Russian passport in 2015 according to the presidential decree published on the official Kremlin website, was detained near his house in Kiev.
"Currently, the preliminary qualification of the criminal offence (...) is treason," a representative of the prosecutor's office, Igor Ponochovny, told journalists during a joint briefing with the SBU official.
SBU deputy chief Viktor Kononenko accused Vyshynsky of travelling to Crimea in 2014 to carry out "subversive" reporting to justify the peninsula's annexation. He said Vyshynsky later collaborated with separatist groups in eastern Ukraine.
"For his work Vyshynsky was awarded the medal "For the return of Crimea" by the decree of Putin," said Kononenko.
Vyshynsky, who has been heading up RIA Novosti's website for readers in Ukraine, received ample funding to carry out his work, said the SBU deputy chief.
"Today we can say that he received 53,000 euros a month for such activities," said Kononenko. "The money was subsequently distributed to the groups and people involved. That was the reason for the searches."
- OSCE 'concerned' -
Russia immediately condemned the searches and detention of Vyshynsky.
"If it is the case that the actions of Ukrainian law enforcement bodies are somehow connected with the professional work of these media organisations, that would be outrageous and scandalous," Kremlin spokesman Dmitry Peskov told journalists.
"Any action against Russian media is unacceptable," he said, adding that the Ukrainian government has repeatedly "put pressure on freedom of speech" in the country.
He warned that Moscow could take measures in response, saying the Kremlin "will vigorously defend the interests of Russian media to the fullest extent."
The Organisation for Security and Co-operation in Europe (OSCE) representative on Freedom of the Media Harlem Desir also raised concerns over the raid.
"The fight against propaganda must not fall short of international standards and should not represent disproportionate interference in media activities," Desir said in a statement.
The director of the public media conglomerate Rossiya Segodnya, the parent company of RIA Novosti, called for the release of Vyshynsky and said the "persecution" of Russian media in Ukraine must end.
"This is not the first time when the Kiev regime tramples on fundamental rights and freedoms by persecuting journalists," Dmitry Kiselev was quoted as saying by RIA Novosti.
Margarita Simonyan, the editor-in-chief of the RT television channel that is also part of the same media group and shares office space with RIA Novosti in Kiev, linked the raids and arrest to the opening of the bridge.
"Kiev decided to take revenge on us for the Crimean bridge," she said on her Twitter feed.
- Media tensions -
Up to 15 journalists work for RIA Novosti in Ukraine. Some of them are responsible for transmitting information to its headquarters in Moscow while others, led by Vyshynsky, are in charge of the ria.com.ua website for Ukrainian readers.
Kiev authorities regularly accuse Russia of using its public media, including television stations which are very popular in Russian-speaking regions of Ukraine, to encourage separatist sentiment and undermine confidence in the current pro-European government.
Ukraine had already banned the distribution of the main Russian TV channels on its territory, blocked the main Russian social networks and expelled several Russian journalists from its territory.
Ukraine ranks 101st out of 180 countries in the 2018 World Press Freedom Index, compiled by Reporters Without Borders (RSF). Russia came in at 148.
Ukrainian journalists wait in front of closed door to the Kiev office of Russia's state-run RIA Novosti news agency
Kremlin spokesman Dmitry Peskov said there could be retaliatory measures