Ukraine's President Volodymr Zelensky has accused Russia of "doing everything it can" to prevent a meeting with Vladimir Putin to try to end the war.
US President Donald Trump has sought to bring the two leaders together, but he said on Friday "that's like oil and vinegar... they don't get along too well".
Russian Foreign Minister Sergei Lavrov said Putin was ready to meet Ukraine's leader "when the agenda is ready for a summit, and this agenda is not ready at all", accusing Zelensky of saying "no to everything".
After an intensive week of diplomacy, in which Trump first met Putin in Alaska and then Zelensky with European leaders in Washington, the US president said the war was turning out to be the most difficult he had tried to stop.
Trump said after a call with the Russian leader on Monday that he had begun arrangements for a Putin-Zelensky summit that he would join afterwards.
