WASHINGTON—North Korea has doubled the size of its facility for enriching uranium in recent years, according to the United Nations’ top nuclear inspector, who voiced doubt that a diplomatic agreement can end leader Kim Jong Un’s weapons programs.

In an interview with The Wall Street Journal on Monday, Yukiya Amano, director general of the International Atomic Energy Agency, described North Korea as rapidly advancing its capacity to produce nuclear weapons on two fronts: the production of plutonium at its Yongbyon nuclear facility...