Talks between Ukrainian and U.S. officials on proposals for a settlement of the war in Ukraine ended after more than five hours on Sunday and are set to resume Monday, a Ukrainian presidential adviser said.

"They went on for more than five hours and ended for today with an agreement to resume tomorrow morning," adviser Dmytro Lytvyn told reporters in a WhatsApp chat.

Lytvyn said Ukrainian President Volodymyr Zelenskyy would comment on the talks on Monday once they were completed.