The Kremlin on Friday warned Ukraine's leader Volodymyr Zelensky to enter negotiations "now" or lose more territory after the US sent Kyiv a peace proposal heeding to many of Moscow's demands.

"The effective work of the Russian armed forces should convince Zelensky: it is better to negotiate and do it now rather than later," Kremlin spokesman Dmitry Peskov said.

"The space for the freedom of decision-making is shrinking for him as territories are lost during offensive actions by the Russian army," he added, while saying Moscow had not officially received the US plan.