The Russian Orthodox Church on Friday slammed its Istanbul-based rival for sending representatives to Ukraine and vowed to retaliate as the two churches are locked in negotiations over Ukraine's religious future.
The jab comes a week after Russia's Patriarch Kirill met with Bartholomew I of the Constantinople Church, who is expected to rule in the coming months on a Ukrainian appeal to cut spiritual ties with Moscow.
Patriarch Kirill, who has strong connections to the Kremlin and is seen as an ally of Russian President Vladimir Putin, is determined to prevent this from happening.
Writing on Russia's popular Telegram app, the Russian church spokesman Vladimir Legoyda called this "nothing more than an unprecedented brutal invasion of the canonical territory of the Moscow Patriarchy".
"Such actions cannot remain unanswered," Legoyda said.
He added that Constantinople did not inform Moscow prior to the decision.
The Orthodox church in Ukraine is split between a branch whose clerics pledge loyalty to Moscow and one that is overseen by the unrecognised Kiev-based Patriarch Filaret.
Ukraine's Moscow loyal church joined Legoyda's sharp criticism and put the blame directly on Bartholomew.
"The responsibility for possible negative consequences of this step lies with the Constantinople patriarchy," it said in a statement published on its website.
Earlier the Constantinople Church said it had sent two bishops "in preparation to gift the autocephaly (the right to form a church) to the Orthodox Church of Ukraine."
Ukraine's appeal to Patriarch Bartholomew comes against the backdrop of an ongoing, four-year conflict between Kiev and Russian-backed rebels in eastern Ukraine that made many Ukrainians turn away from the Moscow church.
Kiev hailed last week's summit between Bartholomew and Kirill as a success, with the country's foreign minister calling the news from Turkey as "historic".
Were Moscow to lose control of the Ukrainian church, it would be seen as a blow to the prestige of the Russian Orthodox Church and Russian spiritual influence in general.
Bartholomew I, known as Archbishop of Constantinople, New Rome and Ecumenical Patriarch, is regarded as the "first among equals" of the world's estimated 300 million Orthodox Christian believers.
His term in office has been marked by rocky relations with the Russian Orthodox Church, which usually gives short shrift to the idea he is the spiritual leader of Orthodox believers.