Ukraine's SBU security service and police raided a 1,000-year-old Orthodox Christian monastery in Kyiv early on Tuesday as part of operations to counter suspected "subversive activities by Russian special services".
The sprawling Kyiv Pechersk Lavra complex - or Kyiv Monastery of the Caves - is a Ukrainian cultural treasure and the headquarters of the Russian-backed wing of the Ukrainian Orthodox Church that falls under the Moscow Patriarchate.
The Russian Orthodox Church, whose head Patriarch Kirill has strongly supported Moscow's military actions in Ukraine, condemned Tuesday's raid as an "act of intimidation".
The Security Service of Ukraine (SBU) said in a statement: "These measures are being taken ... as part of the systemic work of the SBU to counter the destructive activities of Russian special services in Ukraine."
It said the search was aimed at preventing the use of the cave monastery as "the centre of the Russian world" and carried out to look into suspicions "about the use of the premises ... for sheltering sabotage and reconnaissance groups, foreign citizens, weapons storage".
The "Russian world" concept is at the centre of President Vladimir Putin's new foreign policy doctrine that aims to protect Russia's language, culture and religion. It has been used by conservative ideologues to justify intervention abroad.
The SBU did not say what the result of Tuesday's raid was.
The SBU, police and National Guard also searched on Tuesday two other monasteries and the headquarters of the Moscow Patriachate's diocese in western Ukraine, the SBU's branch in the Rivne region said in a statement posted on Facebook.
The raid will further sour already very tense relations between Russian and Ukrainian Orthodox Christians.
"Like many other cases of persecution of believers in Ukraine since 2014, this act of intimidation of believers is almost certain to go unnoticed by those who call themselves the international human rights community," said Vladimir Legoida, a spokesperson for the Russian Orthodox Church.
Last Friday the SBU said it had charged a senior clergyman from the western Vinnytsia region with attempting to distribute leaflets justifying Russia's invasion of Ukraine.
In May, the Ukrainian Orthodox Church of the Moscow Patriarchate ended its ties with the Russian Church over the latter's support for what Moscow calls its "special military operation".
Ukraine says the full-scale invasion began an unprovoked war of aggression.
A 2020 survey by the Kyiv-based Razumkov Centre found that 34% of Ukrainians identified as members of the main Orthodox Church of Ukraine, while 14% were members of Ukraine's Moscow Patriarchate Church.
In 2019, Ukraine was given permission by the spiritual leader of Orthodox Christians worldwide to form a church independent of Moscow, largely ending centuries of religious ties between the two countries.