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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Hyunseung Lee, a former soldier who fled North Korea, told BI that those sent to fight against Ukraine have "zero relationship with modern warfare."

The action suggested North Korea will likely maintain its its confrontational stance against the U.S. for now, even though President Donald Trump said he intends to reach out to North Korean leader Kim Jong Un.

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 test-fired strategic cruise missiles on Saturday the official Korean Central News Agency reported, marking a third launch this month as Donald Trump returned to power in the US.

North Korea has tested sea-to-surface strategic cruise guided missiles, Pyongyang's state media reported on Sunday. State-controlled KCNA news agency quoted the country's Missile Administration as stating the test-fire,