President Trump is expected to receive a letter from the North Korean leader, Kim Jong-un, hand-delivered by his envoy, on Friday afternoon, as the president and his secretary of state, Mike Pompeo, voice optimism about efforts to reinstate a canceled summit meeting with Mr. Kim.

But on Thursday, Mr. Kim also welcomed a top Russian official to Pyongyang, offering a reminder that competing powers could still buffet the White House’s effort to salvage the meeting.

Mr. Pompeo, who met over two days with a senior emissary of Mr. Kim in New York, said on Thursday that he was “confident we are moving in the right direction.”

It would be “nothing short of tragic to let this opportunity go to waste,” Mr. Pompeo said after two and a half hours of discussions with Kim Yong-chol, the former North Korean intelligence chief and top nuclear arms negotiator. “If these talks are successful, they will truly be historic.”