The Vatican’s top ecumenical official has denounced Russian Orthodox support for the war in Ukraine, saying that it imperils ecumenical relations.

Cardinal Kurt Koch, the prefect of the Dicastery for Christian Unity (formerly known as the Pontifical Council for Christian Unity), told the German journal Die Tagespost: “In my opinion it is heresy that the Patriarch dares to legitimize the brutal and absurd war in Ukraine for pseudo-religious reasons.”

The support of the war by Patriarch Kirill was the reason why a proposed “summit meeting” between Pope Francis and the Russian prelate, scheduled for this month, was shelved, Cardinal Koch said. He explained that a joint statement against persecution of Christians—the proposed object of that meeting—would have been “completely implausible” in light of the Patriarch’s support for a war that pitted Christians against Christians.

Cardinal Koch said that he still hopes for progress in relations between Rome and Moscow, saying that “you must never close the doors.” But he expressed frustration with the process, charging that the Russian Orthodox Church had “instrumentalized” a previous meeting between the Pope and the Patriarch, exploiting it to further Moscow’s cause in its dispute with the Ecumenical Patriarchate of Constantinople.