President Vladimir Putin has called on Ukrainian troops in the Russian region of Kursk to "surrender" as Ukraine's President Volodymyr Zelenskyy accused the Russian leader of seeking to sabotage a ceasefire initiative.

US President Donald Trump urged Putin to spare the lives of the Ukrainian troops as he said his envoy had held "productive" talks with Russia's leader on a proposed 30-day ceasefire.

"We are sympathetic to President Trump's call," Putin said in remarks broadcast on Russian television.

"If they lay down their arms and surrender, they will be guaranteed life and dignified treatment," Putin said.

'Horrible massacre'

Earlier in the day, Trump said he had "very good and productive" call with Putin, expressing optimism about ending the war in Ukraine.

Trump said "thousands" of Ukrainian troops were "completely surrounded by the Russian military, and in a very bad and vulnerable position".

"I have strongly requested to President Putin that their lives be spared. This would be a horrible massacre, one not seen since World War II," Trump said.

Ukraine's military leadership denied the claims. "There is no threat of our units being encircled," Ukraine's General Staff posted on social media.

Zelenskyy gave a more sober assessment in comments to reporters in Kiev. "The situation in the Kursk region is obviously very difficult," he said, while insisting the campaign still had value.

Putin said on Thursday that he had "serious questions" about the proposal and that events in Kursk would influence the next moves towards a ceasefire.

Zelenskyy claimed the Russian leader was seeking to undermine the ceasefire initiative.