China began live-fire exercises in the Gulf of Tonkin on Monday, merely days after Vietnam announced a new line marking what it considers its territory in the body of water between the countries.
According to China's Maritime Safety Administration, the exercises would be focused on the Beibu Gulf area, closer to the Chinese side of the Gulf of Tonkin, and will continue till Thursday evening.
The drills follow an announcement made by Vietnam establishing a baseline used to calculate the width of its territorial waters in the Gulf of Tonkin.
State-run Vietnam News reported that the baseline was in compliance with the UN Convention on the Law of the Sea and would provide "a robust legal basis for safeguarding and exercising Vietnam's sovereignty, sovereign rights and jurisdiction."
Vietnam is yet to respond to the Chinese drills.
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 has been growing aggressive in pursuing those claims, and in October assaulted 10 Vietnamese fishermen near the Paracel Islands, three of whom suffered broken limbs.
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China claims almost the entire South China Sea as its own, though it has not publicly released the exact coordinates of its claim other than a map with 10 dashed lines broadly demarcating what it calls its territory.
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.
Tensions have been particularly high with the Philippines, with regular confrontations between the two countries. In the most recent incident, a Chinese navy helicopter flew within 10 feet of a Philippine patrol plane last week over the South China Sea, near the hotly disputed Scarborough Shoal off the northwestern Philippines.
—With agency inputs