Orthodox leader Ecumenical Patriarch Bartholomew, spiritual leader of some 300 million Orthodox Christians worldwide, has issued an urgent appeal to Russia to end the war in Ukraine.

In a statement, he said the Russian invasion of Ukraine was "beyond every sense of law and morality", and that Ukrainians were entitled "to live freely and to determine their own lives".

The Patriarch called on Russia to "immediately stop any act of violence, anything that spreads pain and death", and to "let reason prevail". 

"Indeed, during the last few days, a tragic humanitarian catastrophe is unfolding in Ukraine," he said.

"A war, which, like any war, is an abominable and reprehensible situation. It is the domination of irrationality over reason, hatred over love, darkness over light, death over life.

"Again, from this position, we address another plea to end the war now! To immediately stop any act of violence, anything that spreads pain and death.

"Let reason prevail, love for fellow human beings, reconciliation and solidarity, the light of the Risen Christ, the gift of life." 

Ukrainian President Volodymyr Zelensky has publicly thanked the Patriarch for his support after they spoke on the phone over the weekend. 

President Zelensky said on Twitter, "Thank you for the warm conversation, Your All-Holiness #Bartholomew. Your words are like hands that hold us up in this difficult time.

"Ukrainians feel the spiritual support and strength of your prayers. We hope for the soonest peace."

It was Patriarch Bartholomew who granted autocephaly to the Orthodox Church of Ukraine in 2019, making it independent from the Russian Orthodox Church. 

Metropolitan Epiphanius, Primate of the Orthodox Church of Ukraine, has also made repeated calls on social media for an end to Russian aggression. 

He praised the "unbreakable" and "invincible spirit" of the people of Ukraine in continuing to repel Russian forces. 

"Every hour of our resistance inspires more and more people around the world to support Ukraine," he said in a message posted to social media on Monday.

The Metropolitan likened Vladimir Putin to Hitler and warned that "the whole world is in danger" after the Russian president ordered the country's nuclear deterrent forces to be put on high alert.

"Therefore, the task is not only of our country, but all the countries of the world and all the people of goodwill to stop Putin now. Act, and act immediately. I urge world leaders to do this," said Epiphanius.