By Olena Harmash and Dan Peleschuk

KYIV (Reuters) -Russian forces are preparing for a new offensive against Ukrainian defenders in late May or summer, but Kyiv has a clear battlefield plan of its own, President Volodymyr Zelenskiy said on Sunday.

The Ukrainian leader said at a two-hour news conference it was vital for Kyiv and its Western allies to remain united, predicting two difficult months ahead followed by a fresh Russian assault.

“Russia will prepare counteroffensive actions at the start of summer or end of May. If they are able to. They will prepare,” Zelenskiy told reporters in Kyiv.

“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.”

Ukraine marked the second anniversary of Russia’s full-scale invasion on Saturday as Moscow’s troops bear down along a sprawling front line and problems pile up from shortages of artillery shells to a lack of fresh manpower.

Zelenskiy said 31,000 Ukrainian soldiers had been killed since February 2022, giving the first official toll for more than a year.

He 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.

“They know that we need support within a month,” he said, adding that he felt “positive” about the prospect of long-range missiles being supplied to Kyiv by its partners.

‘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 shakeup 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.

BATTLEFIELD LOSSES

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

“31,000 Ukrainian troops have been killed in this war. Not 300,000, not 150,000 … (Russian President Vladimir) Putin is lying there … But nevertheless, this is a big loss for us.”

He declined to give a figure for the number of wounded, saying that would help Russian war planners. The Russian foreign ministry immediately rejected the estimate of Ukrainian military losses as untrue.