A ceasefire declared by Russian President Vladimir Putin to mark Orthodox Christmas has apparently failed to take hold in Ukraine.

Putin said the pause would last for 36 hours until midnight Saturday, Moscow time.

But the Ukrainian General Staff said on Saturday that Russian forces have continued their attacks using tanks and artillery.

Governor Pavlo Kyrylenko of the eastern region of Donetsk wrote on social media that two civilians were killed in attacks on Friday night.

In the southern Kherson region, Governor Yaroslav Yanushevych told local TV on Saturday that the Russian military has shelled Kherson city.

Russia's defense ministry said its forces fired back to counter Ukrainian attacks. One media report acknowledged that the retaliatory fire "eliminated more than 20 Ukrainian servicemen, a tank" and other vehicles.

Ukrainian President Volodymyr Zelenskyy has rejected Putin's unilateral ceasefire order, branding it "a cover" to allow Russian forces to regroup.

Ukraine's presidential adviser Mykhailo Podolyak tweeted on Saturday that Russia "fires along the entire contact line, but again whines: 'Why does Ukraine not support our false proposal?'"