In an official statement carried by the Korean Central News Agency, North Korea said: "As of now, inter-Korea relations enter a state of war and all matters between the two Koreas will be handled according to wartime protocol".
The 1950-53 Korean War concluded with an amistice, not a peace treaty. The two Koreas have thus technically remained at war for decades.
The North has been issuing dire warnings, in the wake of joint military exercises between South Korea and the US. Although South Korea has largely dismissed the latest official pronouncement as an old threat, there are concerns that the tension might escalate.
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