North Korea appears to be building a new uranium-enrichment plant in its main nuclear complex, the United Nations’ nuclear watchdog warned this week, the strongest sign yet that the country’s leader, Kim Jong-un, plans to grow its nuclear weapons supply.

       Until now, experts and the International Atomic Energy Agency had identified two undeclared uranium-enrichment plants in North Korea. One is in Yongbyon, North Korea’s main nuclear complex, 62 miles north of its capital, Pyongyang. The other plant is in Kangson, just outside Pyongyang.

       But in his report to the I.A.E.A.’s board of governors in Vienna on Monday, its director general, Rafael Grossi, said that his agency was “monitoring the construction of a new building at Yongbyon which has dimensions and features similar to the Kangson enrichment plant.”

       Mr. Grossi’s statement provided no further details about the new facility. But it marked the strongest indication to date that North Korea is building another uranium-enrichment plant at Mr. Kim’s repeated exhortation to his country to expand its nuclear arsenal.

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       ？Western officials and analysts are closely monitoring North Korea’s facilities because its growing nuclear capabilities, as well as its newly forged alliance with Russia, ？could reinforce Mr. Kim’s leverage should he ？return to the negotiating table with the United States or South Korea.

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       North Korea has been producing both types of atomic bomb fuel？ — plutonium and highly enriched uranium？ — for years.

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