"All members of VSIL are concerned about the recent tensions caused by the Chinese side in Vietnam's exclusive economic zone and southeastern continental shelf in the South China Sea," Nguyen Ba Son, president of the society, wrote in an open letter on August 24 to his counterpart in the China Society of International Law (CSIL), Huang Jin.

Son was referring to the oil survey vessel Haiyang Dizhi 8 and its escort vessels illegally reentering Vietnamese waters near the Vanguard Bank in the southern part of the South China Sea, known as the East Sea in Vietnam, on August 13.

The vessels had left Vietnam's exclusive economic zone (EEZ) and continental shelf on August 7 after trespassing for a month.

The letter said the area where the survey vessel had been conducting its illegal activities since early July lay entirely within Vietnam's EEZ and continental shelf as determined on the basis of Articles 57 and 76 of the United Nations Convention on the Law of the Sea (UNCLOS).

"Obviously, this is neither an overlapping nor a disputed area between Vietnam and China."

The activities of Haiyang Dizhi 8 "have seriously violated provisions of UNCLOS and Vietnam's laws regarding sovereign rights and jurisdictions in Vietnam's exclusive economic zone and continental shelf despite opposition from Vietnam.

"I hope CSIL members will provide international legal advice and recommend that the Chinese government should immediately end its violation of international law and withdraw the survey vessel and its escort vessels."

Foreign Ministry spokeswoman Le Thi Thu Hang said at a press briefing last Thursday that Vietnam had on several occasions called on China to withdraw its ships.

The U.S. State Department said last week it was deeply concerned about China's interference in oil and gas activities in waters claimed by Vietnam, and the deployment of the vessels was "an escalation by Beijing in its efforts to intimidate other claimants out of developing resources in the South China Sea."

In a tweet on August 20 U.S. National Security Advisor John Bolton accused China of "bullying."

The issue of Chinese vessels intruding into Vietnamese waters in the South China Sea was also at the center of the 52nd ASEAN Foreign Ministers' Meeting held in Bangkok, Thailand, between July 29 and August 3.

Vietnam’s Deputy Prime Minister and Minister of Foreign Affairs Pham Binh Minh had denounced China's activities at the meeting, calling them "illegal" and "serious violations of Vietnam's sovereignty and jurisdiction."

His counterparts from the U.S., Japan and Australia also expressed concern over "aggression" and interference with oil and gas activities in the South China Sea.

China claims almost the entire South China Sea, including waters close to Taiwan, Brunei, Malaysia, the Philippines, and Vietnam.