Military analysts say Russia is close to ejecting Ukrainian forces from their months-old foothold in the western Russian region, prompting U.S. President Donald Trump to warn that thousands were "completely surrounded".

In a statement on social media after being briefed by his top general, Zelenskiy said Kyiv's troops were not encircled in Kursk but that Moscow was accumulating forces nearby for a separate strike.

"This indicates an intention to attack our Sumy region," he said. "We are aware of this, and will counter it."

"I would like all (our) partners to understand exactly what Putin is planning, what he is preparing for, and what he will be ignoring."

Russian leader Vladimir Putin said on Thursday he supported in principle Trump's proposal for a 30-day ceasefire with Ukraine, but would fight on until several crucial conditions were worked out.

On Saturday, British Prime Minister Keir Starmer told a meeting of about 25 European leaders and other allies that they would need to increase pressure on Putin to accept a ceasefire.

"The build-up of Russian forces indicates that Moscow intends to keep ignoring diplomacy," Zelenskiy added. "It is clear that Russia is prolonging the war."

In his statement, he also said the battlefield situation near the strategic eastern Ukrainian city of Pokrovsk had "stabilised", and that Ukraine had successfully used a new domestically-produced long-range missile in combat.

Kyiv is seeking to expand its domestic defence industry to wean itself off Western allies who have provided critical artillery, air-defence and long-range strike capabilities.

Ukraine's new "long Neptune" missile has a range of 1,000 kilometres (621 miles), Zelenskiy said.