A Chinese oil survey vessel that has been embroiled in a tense standoff with Vietnamese vessels in the South China Sea left Vietnamese-controlled waters today after more than three months, marine data showed.

The Chinese vessel, the Haiyang Dizhi 8, was speeding away from Vietnam's exclusive economic zone towards China today morning under the escort of at least two other Chinese ships, according to data from Marine Traffic, a website that tracks vessel movements.

China claims almost all the energy-rich waters of the South China Sea but neighbours Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims.

The tension between Hanoi and Beijing escalated when China sent the vessel to conduct seismic surveys in waters off Vietnam in early July.

The foreign ministry in Hanoi has repeatedly accused the vessel and its escorts of violating Vietnam's sovereignty and has demanded that China remove its ships from the area.

The ministry did not respond immediately to a Reuters email seeking comment today.

Police broke up a brief protest in August outside the Chinese embassy in Hanoi over the survey vessel.

"China doesn't want any non-Asean companies to drill for oil in the South China Sea," said Ha Hoang Hop, a member of the International Institute for Strategic Studies.

Hop also said China only withdrew the vessel shortly after the oilrig Hakuryu-5 completed its drilling at Vietnam's Block 06.1, which is operated by Russian state oil firm Rosneft.

Chinese coastguard ships have also been operating within the oil block since the standoff began, the Marine Traffic data showed.

Rosneft did not respond immediately to an email requesting comment.

"China is determined to pressure Vietnam to end joint oil exploration and production with foreign partners in the area," said Hop, who is also a visiting senior fellow at the Singapore-based ISEAS-Yusof Ishak Institute.

PetroVietnam told Spanish energy firm Repsol last year to halt an offshore oil project under pressure from China, and a unit of Rosneft expressed its concern that its recent drilling could upset China.

Vietnamese President and Communist Party chief Nguyen Phu Trong called last week for restraint in the South China Sea, saying Vietnam should "never compromise" on its sovereignty and territorial integrity.

Chinese defence minister Wei Fenghe said on Monday the South China Sea is an inalienable part of China's territory. "We will not allow even an inch of territory that our ancestors have left to us to be taken away," Wei said.

The Chinese vessel had been thought to have stopped the surveys when it headed away from Vietnam's exclusive economic zone in early August to Fiery Cross Reef, a man-made island controlled by China that was built on a disputed South China Sea reef. Vietnam and the Philippines also have competing claims on the reef.

"It's very likely that China will send an oil rig to drill in the area where the Haiyang Dizhi 8 had conducted seismic surveys in Vietnam's exclusive economic zone," Hop said.

- Reuters