North Korea attempted to sell a form of lithium metal, a key material for developing miniaturized nuclear weapons, to unidentified international buyers last year, according to United Nations investigators tracking dictator Kim Jong Un's weapons-of-mass-destruction programs.

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The attempted sale, documented in a U.N. report this month, has sparked new concern in the Trump administration, Congress and the U.N. about the proliferation threat posed by Pyongyang's growing nuclear- and ballistic-missile programs, congressional officials and nuclear-weapons experts said.

"The U.N. Panel of Expert's report on North Korea provides further evidence that North Korea will stop at nothing to advance its illicit nuclear and missile programs," said Sen. Cory Gardner (R., Colo.) who wrote a new sanctions bill against North Korea last year.

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North Korea's production of the enriched lithium, known as lithium-6, is also seen by nuclear experts as evidence of the country's accelerated efforts to miniaturize a nuclear warhead, potentially for use on ballistic missiles.

Lithium-6 can be used to produce tritium, which is used to flood neutrons into a nuclear device and magnify the explosive power of nuclear detonations, allowing countries to build bombs with smaller amounts of plutonium or uranium, nuclear experts said.

These smaller devices can be affixed to intercontinental ballistic missiles.

"Lithium-6 is ideal, not only for making tritium for boosting fission devices, but also for directly fueling advanced weapons—including thermonuclear bombs," said Henry Sokolski, a former Pentagon official who heads the Nonproliferation Policy Education Center, a Washington think tank.

Lithium also has nonmilitary applications, particularly in producing batteries, lubricants and medicines, nuclear experts said. North Korea is believed to have significant natural deposits of the metal in its soil.

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