SEOUL (Reuters) - North Korean leader Kim Jong Un called for bolstering nuclear forces this year during a visit to the country's nuclear material production base and nuclear weapons institute, state media KCNA said on Wednesday.

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 Korean leader Kim Jong-un has inspected a nuclear-material production base and a nuclear weapons institute, calling for overfulfilling the plan for producing weapons-grade nuclear materials and strengthening the country's nuclear shield,