President Vladimir Putin on Saturday praised the Russian Orthodox Church for supporting Moscow's forces fighting in Ukraine in an Orthodox Christmas message designed to rally people behind his vision of modern Russia.

The Kremlin issued Putin's message after the Russian leader attended an Orthodox Christmas Eve service on his own inside a Kremlin cathedral rather than joining other worshippers in a public celebration.

In his message, accompanied on the Kremlin website by an image of him standing before religious icons, Putin made it clear he saw the Russian Orthodox Church as an important stabilising force for society at a time he has cast as a historical clash between Russia and the West over Ukraine and other issues.

"It is deeply gratifying to note the enormous constructive contribution of the Russian Orthodox Church and other Christian denominations in unifying society, preserving our historical memory, educating youth and strengthening the institution of family," said Putin.

"Church organisations prioritise ... supporting our warriors taking part in the special military operation (in Ukraine). Such massive, complex and truly selfless work deserves sincere respect."

On Friday, Putin ordered a 36-hour ceasefire for the celebrations, but Kyiv rejected it as Moscow's ploy to buy time and regroup and Russian and Ukrainian forces exchanged artillery fire after the announcement.

Many Orthodox Christians celebrate Christmas on Jan. 7, but the Russian Orthodox Church's backing for Moscow's war in Ukraine has angered many Ukrainian Orthodox believers and splintered the worldwide Orthodox Church.

Of 260 million Orthodox Christians in the world, about 100 million are in Russia itself and some of those abroad are in unity with Moscow.

Others are strongly opposed, however, and reject Moscow's assertion that its Feb. 24 invasion last year was an essential pre-emptive strike to defend its own security and that of Russian speakers in Ukraine.

Ukraine has about 30 million Orthodox believers, divided between the Ukrainian Orthodox Church of the Moscow Patriarchate and two other Orthodox Churches, one of which is the autocephalous, or independent, Ukrainian Orthodox Church.

In a service on Friday, Patriarch Kirill of Moscow criticised Ukraine for cracking down on the branch of the Orthodox church with longstanding ties to Moscow.