Tensions between Churches in Ukraine have risen sharply in recent years because of Russia's annexation of Crimea and its incursion into the east of the country. Different Churches are loyal to Moscow and Kiev, and the country's bitter divisions are fought out among them as well as in the political arena. 

Now the country's tortuous ecclesiastical politics have taken another twist with an extraordinary denunciation by the Russian Orthodox Church (ROC) of Ukraine's Greek Catholic Church.

The ROC has condemned the Greek Catholics for comments made by their leader Patriarch Shevchuk regarding the recent procession organised by Ukrainian Orthodox believers loyal to Moscow. Ostensibly designed to celebrate the anniversary of conversion of Ukraine and Russia to Christianity, the march terminating in Kiev was seen by many observers as a Moscow-inspired provocation designed to assert Russian power over Ukraine and its Churches.

Shevchuk said that while he had no problem with a purely religious procession, the "secular aspect" of the march raised concerns. He suggested participants were being "used as a tool" in a way that recalled tactics used at the beginning of Ukraine's civil war: "They used old men, women and even children as a shield for armed criminals... many also fear that those participants are used as a tool for some undeclared goal as it was the case in Donbas in the recent past." Shevchuk said the Moscow Patriarchate of the Ukrainian Orthodox Church was "often used as a tool in the hands of the aggressor" – understood as Russia, though Shevchuk refrained from saying so directly.

The ROC has let loose a furious broadside against Shevchuk and his Church. It said that "even this good endeavor called to overcome divisions in the Ukrainian society met with cynical attacks and unfair accusations on the part of the Ukrainian Greek Catholic Church, which used this event to launch new vicious accusations against the canonical Ukrainian Orthodox Church".

It continued: "These statements by the head of the UGCC, unbecoming not only to an archbishop but also to a Christian in general and aimed to enkindle inter-confessional strife and made in the genre of political denunciation cannot but provoke resentment and disgust in the hearts of the Orthodox faithful."

As so often in the Orthodox world, behind this frenzied assault lies centuries of disharmony. Greek Catholic liturgy and Church organisation is Orthodox in form, but the Church was formed when some Orthodox bishops reconciled with the Roman Catholic Church in 1595. It is regarded as apostate by other Orthodox Churches. The ROC statement broadened into an attack on this "uniatism", saying: "Again and again, contrary to the agreements achieved through great efforts on the high level between the Orthodox and the Catholic Churches, unia reminds of itself as a force sowing enmity and hatred by systematically and consistently impeding a reconciliation between East and West."

In its statement, the ROC sought to bring pressure to bear on the Greek Catholics through referring to the joint declaration by Patriarch Kirill and Pope Francis at their historic meeting in Havana in February. A clause in the declaration regarded as a political victory for Patriarch Kirill – and strongly objected to by Shevchuk – said: "It is today clear that the past method of 'uniatism', understood as the union of one community to the other, separating it from its Church, is not the way to re-establish unity." The ROC said Greek Catholics had "failed to listen to this call" and were continuing to "deepen the confrontation in Ukrainian society".

It called for the issue to be an "emergency task" for the forthcoming Orthodox Catholic dialogues planned for September in Italy.

Complicating all this is the continuing rivalry between the ROC and Ecumenical Patriarch Bartholomew. He is based in Istanbul, which as Constantinople and Byzantium was the capital of the Eastern Roman Empire, and his ancient office carries enormous prestige though his Church is tiny. The ROC has been highly critical of moves towards the Greek Catholics by Bartholomew, who in the Orthodox tradition is held to be first among equals, and sees him as attempting to encroach on its own territory. 

Bartholomew wrote to Shevchuk thanking him for his support of the Pan Orthodox Council in Crete last month. Shevchuk had written: "Many of the hierarchs who gathered to reflect on the problems facing the Orthodox Church may be surprised to learn that the Head of the Church, which is often referred to as the greatest obstacle to ecumenical dialogue, will support your sincere prayer that God's presence really felt in all your discussions..." 

He also suggested joint projects for the "purification of memory" and healing the wounds of the past – code for repairing deeply dysfunctional relationships.

In a careful reply, Bartholomew assured Shevchuk of his prayer "for peace and stability in Ukraine". "May Almighty God protect and keep your blessed country," he said.

But Bartholomew's letter will also be seen in Moscow as a provocation. Tensions between the ROC and Istanbul remain high after Moscow struck a damaging blow at Bartholomew by refusing to attend the Pan Orthodox Council at the last minute, throwing the enterprise into disarray and compromising its authority. The ROC is also deeply resentful of an appeal to Bartholomew by Ukraine's Parliament, which called on him to reconcile Ukraine's warring Orthodox Churches and grant them "autocephalous" or self-governing status. It regards Ukraine as its own canonical territory and believes all three competing Ukrainian Orthodox Churches should submit to its rule. 

Russian Orthodox bloggers have been vocal in calling for Bartholomew, whose Church in Turkey numbers only around 3,000 members, to lose his status as first among equals in favour of the giant ROC, by far the largest and wealthiest of the Orthodox Churches. The squabble between the ROC and the Greek Catholics is part of a wider power play, in which the ROC is seeking to assert its ecclesiastical authority over the entire country in defiance of the fervent nationalism stirred up by Russia's behaviour towards it.  

Follow Mark Woods on Twitter: @RevMarkWoods