HANOI, Vietnam: Tensions were again increasing off the coast of Vietnam as Hanoi ordered its navy to shadow a Chinese Coast Guard vessel found in Vietnam's South China Sea exclusive economic zone.

Chinese Coast Guard ships have sailed into Vietnam's energy exploration blocks about 40 times since January 2022.

China considers the area part of its expansive territorial claim in the South China Sea, marked by a "nine-dash line," boundary, which the Permanent Court of Arbitration found in 2016 to has no legal basis.

Although the economic zones are not territorial waters and do not have sailing restrictions under international law, Vietnam and Indonesia have asked China to avoid these areas.

"China is asserting jurisdictional rights to seabed energy resources and (has) used its coast guard to put pressure on regional states," said Ian Storey, a senior fellow at Singapore's ISEAS-Yusof Ishak Institute, as quoted by Reuters.

Russia's state-controlled Zarubezhneft is the operator and a shareholder of one the two blocks, named 06-01; Russia's gas giant Gazprom is a shareholder in the other, 05-03, which is operated by a subsidiary of PetroVietnam, the country's state-owned fossil fuel company, according to the Center for Strategic and International Studies, a think tank in Washington.

The Chinese ship sailed through both blocks, as well as two other Vietnamese energy blocks. The Vietnamese ship Kiem Ngu 278, followed the Chinese vessel, at times closing to distances of just a few hundred meters.

From those blocks, roughly 630 nautical miles from China's Hainan island, the Chinese ships typically return on a direct route to Vanguard Bank where they are stationed, said Reuters. Instead, the Chinese ship later sailed to Malaysia's EEZ.

A spokesperson for China's foreign ministry said the Chinese Coast Guard carries out patrols in the areas under China's jurisdiction in the South China Sea, while respecting international law, and said it was not aware of patrols in energy exploration blocks operated by Russian firms.

Meanwhile, a spokeswoman for Vietnam's foreign ministry said that Vietnam acts in the South China Sea "to protect its legal rights."