"We don't believe anyone, not a single beautiful phrase," Zelensky said in a video address to the nation, adding that Russian troops were regrouping to strike the eastern Donbass region.

"We will not give anything away. We will fight for every meter of our territory," he added.

After another round of peace talks in Istanbul on Tuesday, a Russian negotiator said Moscow would "radically" scale back its attacks around the capital Kyiv and the northern city of Chernigiv.

However, shelling continued during the night.

On Wednesday, US military officials said Russian forces did begin to pull out of the defunct Chernobyl nuclear power site north of Kyiv.

"We think that they are leaving, I can't tell you that they're all gone," said a US defence official speaking on condition of anonymity.

Zelensky said in his video address that any retreat by Russian forces was "the result of the work of our defenders."

Zelensky said that as the centre for the global fight for freedom, Ukraine has the right to demand weapons from the international community, including tanks, planes, artillery systems.

"Freedom must be armed as well as tyranny," he said.

In a separate development, the Russian defence ministry announced a local ceasefire Thursday to allow civilians to be evacuated from Ukraine's port of Mariupol, which has been under siege for weeks, left without water, food and power.