BEIJING—North Korean leader Kim Jong Un arrived in China, North Korea’s main backer, as he sent mixed signals to the U.S. about his willingness to give up his nuclear weapons ahead of a possible second summit with President Trump.

Mr. Kim began his China visit on Monday at the invitation of Chinese President Xi Jinping and plans to stay in the country through Thursday, official North Korean and Chinese news agencies reported early Tuesday. Mr. Kim traveled by train accompanied by his wife, Ri Sol Ju, and senior officials,...