Baselines are a sensitive subject in the South China Sea - where China, Vietnam and other countries in the region including the Philippines, Malaysia, Brunei and Taiwan, have conflicting claims.
China claims almost the entire South China Sea as its own, though it has not publicly released exact coordinates of its claim other than a map with 10 dashed lines broadly demarcating what it calls its territory.
China's Maritime Safety Administration said the live-fire exercises would be focused on the Beibu Gulf area, closer to the Chinese side of the Gulf of Tonkin, and would run through to Thursday evening.
China's military has started live-fire exercises in the Gulf of Tonkin - after Vietnam published a map defining what it considers its territory in the body of water between the two countries.
China and Vietnam have long had a maritime agreement governing the Gulf of Tonkin, but have been locked in competing claims over the islands and maritime areas.
