Russia vowed on Friday to defend Russian church believers in Ukraine from any illegal activity against them following Kiev's moves towards a historic split from the Russian Orthodox Church.
Ukraine wants to establish a national church, free from its traditional ties to Russia, which it says is a vital step to tackling Russian meddling in its affairs. Moscow opposes the move, arguing it would cause a schism in Orthodox Christianity.
The Kremlin's comments could inflame tensions between Kiev and Moscow, whose relations collapsed following Russia's annexation of Crimea in 2014 and the outbreak of a Moscow-backed separatist insurgency in eastern Ukraine.
Critics of Ukraine's plan for an independent church say it could lead to violence and forced takeovers of churches loyal to the Russian Orthodox Church.
President Vladimir Putin's spokesman Dmitry Peskov laid out the Kremlin's stand.
"In the event that the events which are developing take the course of illegal activities, then of course, just as Russia defends the interests of Russians and Russian speakers - and Putin has spoken about this many times – Russia will defend the interests of the Orthodox," he told reporters. "This is an absolutely grounded and absolutely understandable position."
Peskov said the defence would consist exclusively of political and diplomatic measures. However, to Kiev his comments were uncomfortably close to the language used in the run-up to the Crimean annexation and the separatist rebellion.
"We have heard similar messages on the 'protection of the Russian-speaking population' from the Russian Federation as justification for its aggression against Ukraine," Ukrainian Foreign Ministry spokeswoman Mariana Betsa tweeted.
Ukraine secured approval on Thursday from a synod in Istanbul, seat of the global spiritual leader of Orthodox Christians, to establish what is known as an "autocephalous", or independent, church.
The next step is for Ukraine to reunite its various strands of Orthodox faith in that new church, which includes deciding the fate of church buildings and monasteries, some of which are aligned to the Russian Orthodox Church.
"I urge against provocations and speculation," Ukraine's Interior Minister Arsen Avakov wrote on Twitter.
"The Ministry of Internal Affairs will ensure security and law and order. If there is a need to prevent extremism and religious hatred, it (the ministry) will act rigidly - and let it not come as an unexpected surprise for the 'hotheads'!"