Ukraine's president urged civilians on Saturday to evacuate the frontline Donetsk region, the scene of fierce clashes with the Russian military, as Kyiv called on the Red Cross and UN to gain access to its soldiers being held by Moscow's forces.

The eastern Donetsk region has faced the brunt of Russia's offensive since its assault on Kyiv failed weeks into the invasion launched on February 24.

President Volodymyr Zelensky warned in his daily address that thousands of people, including children, were still in the region's battleground areas, with six civilians killed and 15 wounded on Friday, according to the Donetsk governor.

"There's already a governmental decision about obligatory evacuation from Donetsk," Zelensky said, underscoring authorities' calls to leave the besieged region in recent weeks.

"Leave, we will help," Zelensky said. "At this stage of the war, terror is the main weapon of Russia."

Official Ukrainian estimates put the number of civilians still living in the unoccupied area of Donetsk at between 200,000 and 220,000.

A mandatory evacuation notice posted Saturday evening said the coming winter made it a matter of urgency, particularly for the more than 50,000 children still in the region.

"They need to be evacuated, you cannot put them in mortal danger in the winter without heating, light, without the ability to keep them warm," Kyiv's Ministry of Reintegration of Temporarily Occupied Territories said in a statement.

Zelensky, in his address, also once more pressed the international community, especially the United States, to have Russia officially declared a "state sponsor of terrorism".