Patriarch Kirill has called on believers to support pro-Russian "brothers" during Moscow's offensive in eastern Ukraine.
In a sermon last year, he said that dying in Ukraine "washes away all sins".
On February 24 last year, Putin sent troops to Ukraine, saying the fellow Orthodox Christian country needed to be "demilitarised."
In recent months, his army has suffered a series of military setbacks on the ground in the Western-backed country.
Putin has unilaterally ordered his forces to pause attacks for 36 hours for the Orthodox Christmas.
But journalists heard both outgoing and incoming shelling in the frontline city of Bakhmut in eastern Ukraine after the time when the Russian ceasefire was supposed to have begun.
