KIEV -- Ukrainian President Volodymyr Zelensky said Saturday that Kiev has proposed to hold a new round of peace talks with Moscow next week.

Rustem Umerov, who headed the Ukrainian delegation in the previous two talks in Istanbul, had sent the Russian side the offer to hold the meeting next week, Zelensky said in his evening address to the nation.

"The momentum of the negotiations must be stepped up," Zelensky said."Everything should be done to achieve a ceasefire."

Umerov, the former defense minister, was just appointed by the Ukrainian president as secretary of the National Security and Defense Council on Friday.

Russia on Tuesday rejected US President Donald Trump's 50-day ultimatum to agree to a Ukraine ceasefire, dismissing the threat of"severe tariffs" as unacceptable.

Russian Deputy Foreign Minister Sergey Ryabkov emphasized that Moscow favors a diplomatic resolution to the Ukraine conflict and is ready to negotiate.