Moscow has promised to defend Orthodox believers in Ukraine while Kiev accuses its neighbour of preparing for "religious war" after the Istanbul-based Ecumenical Patriarchate's decision to recognise the independence of the Ukrainian Orthodox Church.
The Patriarchate's announcement on Thursday is set to drive a further wedge between Kiev and its Soviet-era master.
Relations broke down during the Maidan uprising of 2014 followed by Moscow's annexation of Crimea and ongoing conflict with Russia-backed separatists in eastern Ukraine.
"It's a historic event but one that carries a risk of serious problems," a top government security official told AFP of the decision on condition of anonymity.
The Ukrainian Church is currently split into separate bodies, one technically overseen by the head of the Russian Orthodox Church, Patriarch Kirill.
But the Kiev government considers this unacceptable given the conflict in the east that has claimed more than 10,000 lives.
The independence decision was met with fury by the Russian Orthodox Church, which could lose control of thousands of parishes if they decide to split off and join the new independent church.
The Russian Orthodox Church denounced the decision as a "catastrophe".
Kremlin spokesman Dmitry Peskov meanwhile said that "Russia, as it protects the interests of Russians and Russian speakers... protects the interests of Orthodox believers".
This protection would be "exclusively political and diplomatic," he said.
But this promise is unlikely to reassure Kiev, as Moscow has justified its actions in Crimea and the east by saying it is acting to protect ethnic Russians.
- Russia wants 'to destabilise' -
"The Kremlin's objective is to instigate a religious war in Ukraine," Ukrainian President Petro Poroshenko said Thursday.
"If you see people who are calling to seize a monastery or a church, know that they are agents of Moscow," he said.
Before the independence decision, an influential Moscow cleric warned that parishioners would not hand over churches to a new Orthodox institution willingly.
Kiev has assured parishes that they will be free to decide whether they want to join the newly recognised unified Church or to stick with the status quo.
But the branch of the church loyal to Moscow -- which has more parishes than its Kiev-affiliated equivalent but fewer worshippers, according to the latest surveys -- fears pressure from Kiev and the seizure of property.
Among the most sensitive topics is the future of the landmark Kiev-Pechersk and Pochayiv monasteries, which are currently aligned with Moscow.
Authorities "insist there will no resorting to force, but how otherwise do they plan to transfer our churches and our land to other confessions?" asked Moscow-loyal Archbishop Kliment Vecheria.
"Radical forces... are saying openly that they will not restrain themselves from aggressive acts," he told AFP.
The Ukrainian interior ministry warned Friday against any attempt to "destabilise the situation", vowing that any violence would be met with a harsh response.
Tensions are particularly high ahead of the Defender of Ukraine Day on Sunday -- an official holiday that is associated with nationalist movements.
Many fear a right-wing march that traditionally takes place on the day in the capital will turn into a pitched battle over the Kiev-Pechersk monastery.
Metropolitan Pavlo, of the Moscow branch of the Ukrainian Church, has "invited" worshippers to spend the day at the monastery on Sunday to mark a religious holiday.
"The Russians are going to try to destabilise the situation and will use the Ukrainian far-right to this end," the anonymous Kiev security service official said.
"They need blood and chaos here" to present Ukraine as a "failing state, unable to guarantee the religious rights of its citizens," he told AFP.
"The Russians will attempt a provocation...but we will resist with all our might," he said.