North Korea's nuclear weapons programme is "absolutely non-negotiable," the powerful sister of leader Kim Jong-un said ahead of a visit by Chinese President Xi Jinping, South Korea's Yonhap news agency reported.

"Our status as a nuclear power is absolutely non-negotiable," the sister, Kim Yo-jong, said on Sunday in a statement carried by North Korea's official Korea Central News Agency, adding the North "will not tolerate any threats."

Beijing and Pyongyang announced on Friday that Xi will visit North Korea from June 8 to 9.

The White House said after the American president's visit that Xi and US President Donald Trump reaffirmed a shared goal of denuclearising North Korea.

Beijing has not disclosed details of the discussions but said China is working in "its own way" toward a "political settlement" of the nuclear issue.