Russian President Vladimir Putin stood alone at a midnight service at a Kremlin church as he marked Orthodox Christmas darkened by Moscow's assault on Ukraine.

Putin attended the service at the Cathedral of the Annunciation, originally designed as a church for the Russian tsars.

He stood alone as Orthodox priests in golden robes conducted a ceremony holding long candles, pictures released by the Kremlin showed.

In previous years Putin usually attended Orthodox Christmas services in Russian provinces or just outside Moscow.

The Russian Orthodox Church observes Christmas on Jan. 7.

In a message released by the Kremlin on Saturday, Putin congratulated Orthodox Christians, saying the holiday inspired "good deeds and aspirations."

He also praised the Orthodox Church, whose influential head Patriarch Kirill has fully backed Putin's offensive in Ukraine.

Church organizations are "supporting our soldiers taking part in a special military operation," Putin said, using the official Kremlin term for the offensive in Ukraine.

"Such great, multifaceted, truly ascetic work deserves the most sincere respect," he added.

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 Feb. 24 last year, Putin sent troops to Ukraine, saying the fellow Orthodox Christian country needed to be "demilitarized."

In recent months, his army has suffered a series of military setbacks on the ground in the Western-backed country.

Putin unilaterally has ordered his forces to pause attacks for 36 hours for the Orthodox Christmas.

But AFP journalists heard both outgoing and incoming shelling in the front-line city of Bakhmut in eastern Ukraine after the time when the Russian cease-fire was supposed to have begun.