China and Vietnam have long had a maritime agreement governing the Gulf of Tonkin, but have been locked in competing claims in the nearby South China Sea over the Spratly and Paracel Islands and maritime areas.
China's Maritime Safety Administration said the exercises would be focused on the Beibu Gulf area, closer to the Chinese side of the Gulf of Tonkin, and would run through Thursday evening.
In addition to Vietnam, China's claims overlap with those of the Philippines, Malaysia, Brunei and Taiwan, while Indonesia has also figured in violent confrontations with the Chinese coast guard and fishing fleets in the waters around the Natuna Islands.
Most recently, a Chinese navy helicopter flew within 10 feet (3 meters) of a Philippine patrol plane last week over the South China Sea, near the hotly disputed Scarborough Shoal off the northwestern Philippines.
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.
