North Korean leader Kim Jong Un has called for bolstering nuclear forces this year during a visit to a nuclear material production base and nuclear weapons institute, state media KCNA reported on Wednesday.

North Korean leader Kim Jong Un has inspected a facility that produces nuclear material and called for bolstering the country’s nuclear capability, state media reports.

South Korea 's Yonhap News Agency said North Korea might be using a vertical cold-launch system to fire the missiles. The system, which is commonly associated with submarine- and ship-based weapons, ejects a missile from its container before the missile ignites midair.

North Korea's foreign ministry vowed the "toughest counteraction" against the United States as long as Washington "refuses" the country's sovereignty.

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North Korea has test-fired sea-to-surface strategic cruise guided missiles, the North's state media reported Sunday, quoting Kim as saying that Pyongyang's deterrence means are being perfected.