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 Korea said Sunday it tested a cruise missile system, its third known weapons display this year, and vowed “the toughest” response to what it called the escalation of U.S.-South Korean military drills that target the North.

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North Korea announced it had performed a test of its cruise missile system on Saturday. It was the communist country’s fourth known missile launch since November and the second in less than a week since President Donald Trump ’s inauguration.

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.

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

Kim Jong Un Vs Trump | Kim Jong Un Nuclear | US News | North Korea News As Donald Trump eyes a potential return to the White House, North Korea's Kim Jong Un is ramping up tensions by showcasing his nuclear arsenal through recent missile tests.

North Korea carried out missile tests on Saturday in the latest act of defiance from the hermit kingdom, which has been ramping up its military activity in recent months