HANOI (BLOOMBERG) - Vietnam accused a Chinese oil-surveying vessel and coast guard escorts of territorial violations by widening their activities after entering the country's exclusive economic zone and operating within offshore blocks for three months.

As of Friday (Oct 4), the ship called the Haiyang Dizhi 8 has made several passes through the foreign-owned blocks off the coast of central Vietnam after departing from the Chinese-controlled Fiery Cross Reef on Sept 28, according to satellite tracking data from Marine Traffic.

"The Chinese survey vessel Haiyang Dizhi 8 and its escorted vessels continue, and expand their operations within Vietnam's exclusive economic zone and continental shelf, thus seriously violating Vietnam's sovereign rights," Vietnam's Ministry of Foreign Affairs spokesman Le Thi Thu Hang said at a Hanoi briefing on Thursday.

Vietnam, which shares a long border with its fellow Communist country, stands virtually alone in the region as it pushes back against Beijing amid territorial disputes in the South China Sea, a region containing unexploited hydrocarbons that the US says could be worth US$2.5 trillion (S$3.5 trillion).

Tensions between China and Vietnam have been on the rise since July, when the Chinese state-owned surveyor first began studying the seabed of the southern block in the disputed South China Sea operated by Rosneft.

China has repeatedly warned Vietnam to abandon exploration projects with foreign companies that it says threaten China's sovereignty.

Vietnam has become increasingly isolated in its efforts to push back against China, which is nearing a deal with the Philippines for joint energy exploration in a contested area of the sea and just set up one-on-one talks with Malaysia to settle disputes in the waters.

"Once again, Vietnam demands that China immediately cease its serious violations, withdraw all of its vessels from Vietnam's maritime zones and desist from repeating similar violations," said Hang.