Since North Korea has never recognized the NLL from their point of view any South Korean movements along the NLL are violations they can use as an excuse for a provocation:
The current NLL is depicted with the Blue line and North Korea’s claimed NLL is depicted with the Red line.
North Korea threatened Sunday to take action against South Korea over what it claims are violations of the de facto western maritime border, ratcheting up tensions ahead of a trilateral summit in Seoul.
North Korea’s supreme military leadership instructed its army on Friday to take offensive action against the encroachment upon its sovereignty, North Korea’s Vice Defense Minister Kim Kang-il said, according to the country’s official Korean Central News Agency (KCNA).
“We officially warn that we can never tolerate such continued encroachment on our maritime sovereignty and that we may exercise our self-defensive power on or under the water at any moment,” Kim said in a statement carried by the KCNA.
The statement came hours before South Korean President Yoon Suk Yeol will have back-to-back bilateral talks with Chinese Premier Li Qiang and Japanese Prime Minister Fumio Kishida at the presidential office.
Yonhap
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