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Cold air from the Arctic infiltrates the East Sea, causing the surface water, laden with carbon dioxide, to become denser.

Vietnam claimed the map showed the nine-dash line, Chinese claims on the shoals, islands and lines of maritime control in what Vietnam calls the East Sea.

"There is no guarantee that a shocking incident where a U.S. air force strategic reconnaissance plane is shot down over the East Sea will not happen," the spokesperson said.

"The East Sea territory defence exercise was carried out to conduct our mission to protect our territory, people and property," a South Korean military official said.

Like most North Korean tests, those missiles all fell in the Sea of Japan, which is known as the East Sea in both Koreas.