The meeting with Russia, while preliminary, would signal the start of Mr. Trump’s accelerated timetable for a deal and his seeming determination to conduct negotiations with Russia alone, at least for now.

Ukraine will not take part, Andriy Yermak, the head of the President Volodymyr Zelensky’s presidential office, confirmed Sunday in a post on the Telegram social networking site. He said that Ukraine would prefer to reach a common plan for negotiations with the Trump administration before meeting with a Russian delegation.

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