Beijing demanded on Thursday that Hanoi cease its harassing actions against a Chinese oil rig in waters off an island in the South China Sea and called for dialogue to end the conflict.

Since May 2, Vietnam has carried out intensive disruptions of a Chinese company's normal oil drilling in waters administered by China. China is deeply surprised and shocked, said Yi Xianliang, deputy director-general of the Department of Boundary and Ocean Affairs at the Foreign Ministry, on Thursday.

Yi said the Xisha Islands are inherent territory of China and there are no disputes in this area. The oil rig operation is undertaken by China Oilfield Services Ltd, and it is a normal drilling activity in the coastal waters off the Xisha Islands of China.

The oil rig operation, which is only 31 km from Zhongjian Island, is completely within waters off China's Xisha Islands, and the operation completely falls within the area of China's sovereignty and jurisdiction.

From Saturday to Wednesday, Vietnam dispatched 36 vessels of various kinds that rammed Chinese vessels as many as 171 times.

The Chinese vessels are only government and civilian vessels. But the Vietnamese have many armed vessels deployed to the scene.

At one point, the Chinese side found Vietnamese frogmen 5 meters away from the Chinese government vessels.

Yi said Vietnam's disruptions of the Chinese company's normal activities have seriously violated China's sovereignty, sovereign rights and jurisdiction, gravely affected the order of the oil rig's work and the rig's safety, and in addition caused unnecessary troubles for China-Vietnam ties.

He added that China had to take more security measures in response to Vietnam's disruptions, so as to ensure the order of maritime operation and the safety of navigation.

The conflict remains unresolved just days ahead of an Association of Southeast Asian Nations summit. Chinese analysts said the dispute could add fuel to tensions between the two nations.

Beijing said lines of communication between Beijing and Hanoi "are working well" and they have had contact many times over the past five days.

Li Yong, CEO of China Oilfield Services, which is contracted to drill on the rig, said that a vessel ramming the oil rig "could have disastrous consequences".

Dong Manyuan, vice-president of the China Institute of International Studies, said countries such as Vietnam have "stirred up ... an incident that has upset neighboring nations".

"All of the other countries are watching this situation closely," Dong said.

Su Hao, a professor of Asia-Pacific studies at China Foreign Affairs University, said Vietnam "has misjudged the ongoing rig drilling, treated it with huge hostility and taken irrational measures".

"Hanoi should change its way of thinking into a cooperative and win-win perspective to better implement what the two countries have agreed on," he added.