Russia says ceasefire started in Ukraine

A unilateral ceasefire in Ukraine declared by Russian President Vladimir Putin has officially came into force but the hours leading up to the truce saw renewed fighting, with Kyiv rejecting the move as a trick.

Russia's Channel 1 state television says the ceasefire began at noon Moscow time on Friday "along the entire line of contact" in the conflict.

Reuters could not immediately establish if there had been any lull in fighting.

Putin ordered the 36-hour ceasefire in the 10-month-long war in a surprise move in Thursday, saying it would mark the Russian Orthodox Christian Christmas.

But Ukraine and its Western allies dismissed it, calling it a ploy aimed at giving Moscow time to reinforce troops and equipment along the eastern front.

Earlier on Friday morning - Christmas Eve for Russians and many Ukrainians - Russian shells hit Kramatorsk, a Ukrainian city near the frontline in the industrial Donetsk region that Russia claims as its territory, the city mayor said.

"Kramatorsk is under fire. Stay in shelters," mayor Oleksandr Honcharenko said on social media. He did not give details of damage.

Shortly after the ceasefire was due to come into effect, Russian-backed officials accused Ukraine of shelling the city of Donetsk with artillery, Russia's state-run TASS news agency said.

Despite air raid warnings sounding in several regions, no major air strikes were reported by Ukrainian officials after the ceasefire starting time.

The battles had not previously slowed during the festive season and Russia mounted waves of air strikes over New Year, usually a time of celebration in both Ukraine and Russia.

Russia invaded Ukraine on February 24 last year, starting a war that has killed tens of thousands of people, displaced millions and reduced cities to rubble.

With financial support and weapons from the United States and Europe, Ukraine has driven Russia back from some of its territory but battles are raging over eastern and southern cities.

Among the most intense battlegrounds is the city of Bakhmut, still in Ukrainian hands despite months of battering by Russian forces that have left much of it in ruins.

In Putin's surprise announcement on Thursday, he unilaterally ordered his troops to observe a ceasefire from Friday to run through the Russian Orthodox Christmas Eve and Christmas Day.

Ukrainian President Volodymyr Zelenskiy rejected the idea out of hand, saying the goal was to halt the progress of Ukraine's forces in Donetsk and the wider eastern Donbas region and bring in more of Moscow's forces.

"They now want to use Christmas as a cover, albeit briefly, to stop the advances of our boys in Donbas and bring equipment, ammunitions and mobilised troops closer to our positions," Zelenskiy said in his Thursday night video address.

"What will that give them? Only yet another increase in their total losses."

Ukraine's military General Staff said its soldiers repelled many Russian attacks over the past day, with Moscow focused on trying to take towns in Donetsk, including Bakhmut.

"The enemy is concentrating its main efforts on attempts to establish control over the Donetsk region" without success, the General Staff said in a statement, adding that both Ukraine and Russia had launched multiple air strikes over the past day.

Reuters could not independently verify the latest battlefield accounts.

US President Joe Biden suggested Putin's ceasefire offer was a sign of desperation. "I think he's trying to find some oxygen," he told reporters at the White House.

Russia's ambassador in Washington, Anatoly Antonov, responded on Facebook saying: "Washington is set on fighting with us 'to the last Ukrainian'."