Veteran North Korean diplomats are being sidelined from nuclear talks ahead of the second summit with the United States as recent defections and allegations of spying undermine the trust of leader Kim Jong Un, South Korean officials and experts say.

Kim has purged and replaced many top diplomats and officials who served his father and grandfather with new, younger advisers as he gears up to meet U.S. President Donald Trump in Vietnam next week.

Among the most significant changes, Kim has appointed little-known Kim Hyok Chol to spearhead working-level talks with U.S. nuclear envoy Stephen Biegun.