DANDONG, China—For most of the past decade, a Chinese state-owned company had a joint venture with a North Korean company under sanctions for involvement in Pyongyang’s atomic-weapons program, Chinese corporate and government records show.

China’s Limac Corp. and North Korea’s Ryonbong General Corp. set up a joint venture in 2008 to mine tantalum, niobium and zirconium, minerals that are useful in making phones and computers but also nuclear reactors and missiles.

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