Ukraine's security service said Tuesday it had raided a historic Orthodox monastery in Kyiv over suspected links to Russian agents.
The Kremlin denounced the searches as the latest chapter in Kyiv's "war" against the Russian church.
Located south of the city center, the 11th-century Kyiv Pechersk Lavra is a UNESCO World Heritage site and seat of a branch of Ukraine's Orthodox Church that was formerly under Moscow's jurisdiction.
It cut ties with Russia soon after President Vladimir Putin invaded Ukraine.
The SBU security service said in a statement that "counter-intelligence measures" were carried out as part of work to "counter the subversive activities of the Russian security services in Ukraine."
The intention was to prevent the site from being a "center of the 'Russian world'" or used to hide "sabotage and intelligence groups" and to store weapons, the statement added.
Early Tuesday, a police car was parked outside the Lavra, its roof and glistening golden domes covered with snow, an AFP reporter saw.
Armed officers were seen carrying out ID checks and searching the bags of worshippers before letting them inside.
"Ukraine has long been at war with the Russian Orthodox Church," Kremlin spokesman Dmitry Peskov told reporters.
"We could see this as yet another link in the chain of these military actions against Russian Orthodoxy."
A spokesman for the Russian Orthodox Church denounced the searches as an "act of intimidation" against Ukrainian worshippers.
"We pray for our fellow believers... who are becoming victims of lawlessness and we call on all sympathetic people to do everything possible to stop this persecution," Vladimir Legoyda said on social media.
The SBU said in a separate statement that "security measures" were carried out Tuesday at two other monasteries and the diocese in the region of Rivne in northwest Ukraine.
Russia lost multiple Ukrainian parishes in 2019, when a historic schism fuelled by the Kremlin's land grab of Crimea and backing of a separatist insurgency in eastern Ukraine led to the creation of the Kyiv Patriarchate of the Orthodox Church.