By Olena Harmash and Dan Peleschuk

KYIV (Reuters) -Russia is preparing a new offensive against Ukraine starting in late May or summer, but Kyiv has a clear battlefield plan of its own, President Volodymyr Zelenskiy said on Sunday.

Speaking a day after the second anniversary of Russia's invasion of Ukraine, Zelenskiy said it was vital for Kyiv and its Western allies to remain united and reiterated that Ukraine's victory depends on continued Western support.

"We will prepare for their assault. Their assault that began on Oct. 8 has not brought any results, I believe. We, for our part, will prepare our plan and follow it," Zelenskiy told reporters in Kyiv.

Zelenskiy said that 31,000 Ukrainian soldiers had been killed since February 2022, giving the first official toll in more than a year. The Russian foreign ministry rejected the Ukraine figure as untrue.

Zelenskiy said that troop rotations would be critically important for the war effort and emphasised that Ukraine needed to better prepare its reserve forces.

A New York Times report in August cited U.S. officials as putting the Ukrainian death toll at close to 70,000. The same report said as many as 120,000 Russian troops had died during the war.

The tallies could not be independently verified. Both Russia and Ukraine have often underestimated their military casualties in the war, while exaggerated the losses they claim to have inflicted upon each other.

TIME RUNNING OUT FOR WESTERN SUPPORT

Two years into the war, Moscow's troops bear down along the sprawling 600-mile (960-km) front line in Ukraine's east and south and problems pile up from shortages of artillery shells and the need for longer-range missiles to a lack of fresh troops.

Zelenskiy said he was confident that the U.S. Congress would approve a major new batch of military and financial assistance and that Ukraine needed that decision within a month.

The Ukrainian war effort depended on Western support, he said, adding that the European Union had only supplied 30% of the 1 million ammunition shells that were promised.

Russia secured its biggest battlefield gains since May 2023 this month as it captured the town of Avdiivka, which Ukrainian troops retreated from to avoid being surrounded.

President Vladimir Putin said on Tuesday Russian troops would push farther into Ukraine to build on their success in Avdiivka and on Sunday Russian defence ministry said its forces had taken more advantageous positions near the town.

'THERE IS A PLAN'

On Ukraine's battlefield intentions, Zelenskiy said Kyiv had a clear plan to counter Russian forces, but that he would not disclose details that could compromise it.

"There is a plan, the plan is clear, I can't tell you the details," he said.

Kyiv's troops conducted a much-vaunted counteroffensive last year but were unable to pierce Russia's defensive lines.

Zelenskiy said replacing his popular armed forces chief in a dramatic military shake-up this month was part of his military strategy that would remain under wraps.

The Ukrainian leader said earlier that Kyiv's plans for last year's counteroffensive had ended up "on a desk in the Kremlin" before the operation had even begun but did not say how.

Kyiv hopes to hold a summit in Switzerland this spring to discuss its vision for peace with its allies, he said, adding that the peace blueprint would later be presented to Russia.

"I hope it will take place this spring. We must not lose this diplomatic initiative," he said.