On Saturday, Kim Kye-gwan, a senior aide to Mr. Kim, confirmed that Mr. Trump had sent a personal letter to Mr. Kim for his birthday​. (Mr. Kim is believed to have turned 36 on Wednesday. ​) ​ He also acknowledged that the “personal relations” between Mr. Kim and Mr. Trump were “not bad,” but said Mr. Kim “would not discuss the state affairs on the basis of such personal feelings. ​”

“Although Chairman Kim Jong-un has good personal feelings about President Trump, they are, in the true sense of the word, ‘personal,’” the official said in a statement carried by the North’s official Korean Central News Agency.