Russia's new offensive in eastern Ukraine is going "very cautiously" and will fail because Moscow's forces lack the strength to break through Ukrainian defences, an aide to President Volodymyr Zelenskiy said on Tuesday.

Presidential adviser Oleksiy Arestovych said Rusisan forces were trying to find "sensitive spots" in Ukraine's defences but added: "Their offensive will fail - I give you a 99% guarantee - they simply do not have enough strength."

"The battle for Donbas, which was announced and apparently began yesterday, is under way and is going very cautiously. The battle will not go in Russia's favour," he said on national television.

Russian forces were trying to push through Ukrainian defences along almost the entire front line in eastern Ukraine, Ukrainian officials said on Tuesday.

After being pushed back by Ukrainian forces from an assault on Kyiv, Russia has regrouped to focus on a new offensive in eastern Ukraine.

(Reporting by Pavel Polityuk, Editing by Timothy Heritage)