Pope Francis implicitly accused Russia on Thursday of "armed conquest, expansionism and imperialism" in Ukraine, calling the conflict a "cruel and senseless war of aggression".
The pope, speaking to a delegation of Orthodox leaders from the Istanbul-based Ecumenical Patriarchate, said the conflict had pitted Christians against one another.
Both Russia and Ukraine are predominantly Orthodox Christian but there is an influential Byzantine-rite Catholic minority in Ukraine that owes its allegiance to the pope.
The Eastern and Western branches of Christianity separated in the Great Schism of 1054.
"Reconciliation among separated Christians, as a means of contributing to peace between peoples in conflict, is a most timely consideration these days, as our world is disrupted by a cruel and senseless war of aggression in which many, many Christians are fighting one another," the pope said.
The pope also told his Orthodox visitors, in a clear reference to Russia, that all needed "to recognise that armed conquest, expansionism and imperialism have nothing to do with the kingdom that Jesus proclaimed".
It was the second consecutive day that the Pope spoke about the Ukraine conflict. On Wednesday he condemned the bombing of a crowded shopping centre in the city of Kremenchuk, calling it the latest in a string of "barbarous attacks" against Ukraine.
The Ukraine war has caused deep divisions among world Orthodoxy. The patriarch of the Russian Orthodox Church (ROC), Kirill, has given his full-throated blessing to Moscow's invasion of Ukraine since it began on Feb. 24.
His position has splintered the worldwide Orthodox Church and unleashed an internal rebellion which has led to some local Orthodox Churches once linked to the ROC to sever ties.
Russian President Vladimir Putin, a member of the ROC, has described Moscow's actions in Ukraine as a "special military operation" aimed at demilitarising and "denazifying" the country. The pope has rejected such terminology.
In an interview published last month in an Italian newspaper, Francis said Kirill "cannot become Putin's altar boy". The ROC later scolded the pope over the remark.
Francis was to have met Kirill on June 14 in Jerusalem but the plan was cancelled in April on the advice of Vatican diplomats.
It would have been only their second meeting. Their first, in Cuba in 2016, was the first between a pope and a leader of the Russian Orthodox Church since the Great Schism.
Earlier this month Britain sanctioned Kirill for "his prominent support of Russian military aggression in Ukraine".
Hungary blocked an EU attempt to sanction him.