BEIJING, May 15 - Recent tensions in the South China Sea are a result of Vietnamese instigations and Hanoi must stop all provocative activities as soon as possible, a Beijing-based expert on international studies told Xinhua on Thursday.

Relations between China and Vietnam have undergone a "subtle change" following a series of provocative activities by Hanoi to disturb the normal drilling of a Chinese oil rig in the waters off China's Xisha Islands, also known as the Paracel Islands, said Cao Qun, a researcher with the China Institute of International Studies.

Cao said China Maritime Safety Administration had issued a notice demanding vessels sailing by to avoid crossing into a three-mile radius around the Haiyang Shiyou 981 oil rig, owned by China National Offshore Oil Corporation, to ensure safety.

In disregard of the navigation notice, however, the Vietnamese authorities dispatched a large number of vessels, including armed ones, to waters near the drilling site, ramming China's civilian ships and disturbing the normal operation of the drilling platform, he said.

The Vietnamese authorities also filed a protest, accusing the drilling operation of falling within "disputable waters" and violating the United Nations Convention on the Law of the Sea and the Declaration on the Conduct of Parties in the South China Sea.

Meanwhile, Hanoi has made use of the so-called "collision incident" in the South China Sea incited by the media, "to describe itself a victim bullied by China," Cao said.

But the fact is that the Chinese oil rig operates only 17 nautical miles from China's Xisha Islands and about 150 nautical miles from Vietnam.

It is self-evident that the site falls into China's offshore waters and that the abovementioned operation is completely within the range of the country's sovereignty, sovereign rights and jurisdiction and has nothing to do with Vietnam, Cao said.

The researcher said the Xisha Islands are a part of China's inherent territory and the drilling of the Chinese enterprise in its offshore waters is completely an internal affair that should be free from any outside interference.

"The normal and legitimate drilling in the sea waters under China's jurisdiction is in compliance with the UN Convention on the Law of the Sea and anything but a violation of the Declaration on the Conduct of Parties in the South China Sea as slandered by Vietnam," he said.

Cao added that facts prove that it is Vietnam, not China, that has instigated the tensions in the South China Sea.

"In the face of provocative activities from Vietnam, China had to reinforce security precautions on the site to ensure the normal drilling of its oil rig and the safety of operating equipment and staff," he said.

Calling China's efforts to fend off bumping Vietnamese vessels a "just move", Cao said China has kept utmost restraint in the face of Vietnamese provocations to avoid the escalation of tense situations.

In response to Vietnamese provocations, China used only civilian vessels and chose a diplomatic channel to communicate with the Vietnamese party, asking Hanoi to respect China's legitimate rights and interests and immediately stop further provocative activities, he said.

"However, such kind of restraint has failed to get respect in exchange. Vietnam increased its vessels at the scene and even mobilized some armed ships and frogmen in the meanwhile," he said.

He added that the fishing nets and barricades deployed by Vietnamese vessels not only seriously threatened the safety of China's vessels, drilling equipment and personnel, but also endangered normal navigation security.

"The serial provocative activities have not only seriously infringed upon China's sovereignty, sovereign rights and right of jurisdiction, but will also probably cause serious damage to Sino-Vietnamese relations that are otherwise on the track of enhancing strategically mutual trust and deepening mutually beneficial cooperation," Cao said.

The expert noted that Vietnamese President Truong Tan Sang had paid a visit to China and held talks with Chinese President Xi Jinping in June 2013.

He said both leaders had unanimously agreed at the time that friendship between China and Vietnam is the common wealth of both peoples and both countries should continue adhering to the "good-neighborliness, friendship and all-round cooperation" principle and continue to be "good neighbors, good friends, good comrades and good partners" to each other.

Cao said China and Vietnam had laid out an overall pattern of coordinatively advancing maritime, land and financial cooperation, and ushering in a new future of booming development of bilateral ties, thanks to joint efforts of both leaders.

The working group for joint maritime development established by the two countries has held two rounds of consultations and achieved some positive progresses, he said.

"In this context, both countries should join to cherish and carry forward with the hard-won good momentum and in particular, remain vigilant against any alienation by the United States and Japan based on their strategic interests," Cao said.

He said there is no doubt that Vietnam's latest provocative activities in the South China Sea have had unfavorable influence on its otherwise good interaction with China.

In order to maintain healthy development of bilateral relations and the stability of the South China Sea, the Vietnamese authorities should "return to reason as soon as possible, immediately stop all provocative activities and withdraw all of its vessels and personnel from the operating site of the Chinese oil rig," he said.