Ukraine's Volodymyr Zelensky said on Saturday his forces would observe a surprise Easter truce announced by Russian President Vladimir Putin set to last until midnight on Sunday.

The 30-hour truce would be the most significant pause in the fighting throughout the three-year conflict.

But just hours after the order was meant to have come into effect, air-raid sirens sounded in the Ukrainian capital and Zelensky accused Russia of having maintained its artillery fire and assaults on the frontline.

Also on Saturday, Russia and Ukraine held a large exchange of prisoners, each side saying they had handed back more than 240 captured fighters.

The order to halt all combat over the Easter weekend came after months of efforts by US President Donald Trump to get Moscow and Kyiv to agree a ceasefire. On Friday, Washington even threatened to withdraw from talks if no progress was made.

"Today from 1800 to midnight Sunday, the Russian side announces an Easter truce," Putin said in televised comments during a meeting with the Russian chief of the general staff Valery Gerasimov.

Zelensky responded by saying Ukraine would follow suit, and proposed extending the truce beyond Sunday. But the Ukrainian leader also accused Russia of having already broken its promises. (AFP)