Patriarch Kirill said in his letter to the Russian and Ukrainian presidents that while an earlier peace agreement in Minsk had eased the civil war somewhat, people are still being killed "to this day".
The United Nations estimates more than 6700 people have died in the civil war since April 2014.
He said: "The Minsk agreements stopped the escalation of the conflict, raising hopes for the complete cease-fire between the belligerent sides.
Regrettably, the hostilities, though not as large-scale as before, have been going on to this day. Blood is being shed and people are being killed in Ukraine."
"Old and disabled people and children are the most vulnerable group. There are people who have neither strength nor opportunities to leave their native lands.
"Unfortunately, not all of them survive in this terrible situation. Some of them die under artillery bombardments; others die from hunger and illnesses, and because of the lack of medicaments and necessities."
"In all the churches of the Russian Orthodox Church at every service fervent prayers ascend to God for the return of peace."
The Patriarch also said there must be: "a full cessation of hostilities, strict observance of the Minsk accords and the establishment of direct dialogue between all parties to the conflict."
