Ukraine says 'Battle of Donbas' has begun, Russia pushing in east

Ukraine's army has been bracing for a new Russian assault on its eastern flank since Moscow withdrew its forces from near Kyiv and from Ukraine's north late last month in order to focus on an assault in the Ukrainian region of Donbas.

"We can now say that Russian forces have started the battle of the Donbas, for which they have long prepared," Zelenskiy said in a video address.

Ukraine Security Council Secretary Oleksiy Danilov said in televised comments: "They (Russian forces) began their attempt to start the active phase this morning.

"This morning, along almost the entire front line of the (eastern) Donetsk, Luhansk and Kharkiv regions, the occupiers attempted to break through our defences," he said.

Russia has been bulking up its forces in the east of Ukraine using troops that it pulled out of Ukraine's north and neighbouring Belarus, a close Russian ally.

In a post on Facebook, the Ukrainian armed forces command said that Russia's main military force was concentrating on taking control of the entirety of the Donetsk and Luhansk regions that make up the swathe of land known as the Donbas.

"The second phase of the war has begun... Believe in our army, it is very strong," the Ukrainian president's chief of staff, Andriy Yermak, wrote on the Telegram messaging app.

Russia's reinforcements have set the stage for a protracted battle that military analysts say is certain to inflict heavy losses on both sides as the Russians try to encircle Ukraine's fighters dug in to defend the Donbas region.