Hanoi authorities are considering a baseline marking Vietnam’s territory in the Gulf of Tonkin, Radio Free Asia has learned.

In March, Beijing announced a new baseline demarcating China’s territorial sea – deemed by experts as “excessive” – in the northern part of the gulf. Chinese officials said the baseline was made “in accordance” with domestic and international laws.

Two Vietnamese government sources familiar with the matter told RFA that “relevant offices” in Hanoi are working on a new baseline for the Gulf of Tonkin, called Vinh Bac Bo in Vietnamese. They did not provide further details.

Before China’s announcement on March 1, 2024, neither country registered any baselines – used to measure the extent of the territorial sea and other maritime zones - in the gulf called Beibu in Chinese.

However, Hanoi and Beijing in 2000 signed a Delimitation Agreement to demarcate their shares of the gulf from the mainland of Vietnam and China in the North to the mouth of the gulf in the South. 

China’s newly announced baseline is not thought to immediately affect Vietnam’s sovereignty in already clearly delineated waters but Hanoi is “definitely concerned by China’s unilateral action,” said Carlyle Thayer, emeritus professor at the Australian Defence Force Academy.

“It could result in Chinese pressure to amend the 2000 Delimitation Agreement on Tonkin to expand China’s maritime zone,” the security expert said.

Vietnam’s reaction

Vietnamese foreign ministry on March 14 released a statement calling on Beijing to “respect and abide by the agreement on the delimitation of the territorial seas, exclusive economic zones and continental shelves of the two countries in the Gulf of Tonkin signed in 2000, as well as the 1982 UNCLOS (U.N. Convention on the Law of the Sea).”

Beijing responded saying it was China's “legitimate and lawful right” to determine the territorial sea baseline in the Gulf of Tonkin.

Experts say the new Chinese baseline on average encroaches 20 to 30 nautical miles (37 to 55.5 kilometers) upon international waters. It seriously impedes freedom of navigation through the Qiongzhou Strait, also known as the Hainan Strait.

“While the actual maritime delimitation line between the two countries is unchanged by this unilateral action, China's new declared baselines bump out its internal waters well beyond what UNCLOS permits and do not bode well for future negotiations,” said Isaac Kardon, senior fellow at the Asia Program at the Carnegie Endowment for International Peace. 

Vietnam and China are negotiating a new agreement on fishery cooperation in the Gulf of Tonkin after the previous one expired in 2019 and “China's redesignation of the adjacent maritime areas will certainly complicate such an effort,” according to Kardon.

Vietnamese legal experts have called on Hanoi to define maritime zones for easier management of resources including fisheries.

“I am confident that the Vietnamese government will announce a new baseline for the Gulf of Tonkin and they will do it soon,” said Hoang Viet, a maritime legal analyst.

The United States which promotes freedom of navigation across the world has yet to say anything about the Chinese baseline.

Edited by Mike Firn.