Vietnam navy South China Sea
A Vietnamese naval soldier stands guard at Thuyen Chai island in the Spratly archipelago January 17, 2013Reuters

Tensions escalated in a disputed part of the South China Sea after Vietnam said a Chinese ship rammed two Vietnamese vessels close to where China has moved a massive oil rig.

Hanoi said the collision took place on Sunday, causing damage to two Vietnamese ships and injuring six on board.

"On May 4, Chinese ships intentionally rammed two Vietnamese vessels. Water cannon was used," said Vietnam's Foreign Ministry official Tran Duy Hai.

He told journalists in Hanoi that the Chinese ships plus air support, sought to intimidate Vietnamese vessels.

China responded to the allegations on Thursday when a senior official said China is "deeply shocked" by the Vietnamese "disruption" to its drilling in the South China Sea.

Yi XianLiang, China's Deputy Director General of Boundary and Ocean Affairs, told reporters that Vietnam's ships had rammed Chinese boats 171 times since May 3.

"We cannot tolerate any behaviour that would undermine the safety of our personnel or our rig or drilling operation," he added.

"In the face of Vietnamese disruption, China had to increase its security forces at the scene," he said, confirming that Chinese boats had fired water cannons at Vietnamese vessels.

The dispute between the neighbours began when China moved a massive oil rig into waters that Vietnam claims as its own. Hanoi complained to the United States over the move but China stated that the oil rig was wholly within its own territorial waters.

Both countries have navy and coastguard ships in the vicinity of the Chinese rig.

"No shots have been fired yet," a Vietnamese navy official told Reuters. "Vietnam won't fire unless China fires first," he added.

The United States has criticised the Chinese over the rig's movement in to disputed waters and urged all sides to show caution.

The incident comes after US President Barack Obama visited East Asia to promote his administration's strategic "pivot to Asia". Obama visited US allies Japan and the Philippines who are both involved in territorial disputes with China.

Washington is "strongly concerned about dangerous conduct and intimidation by vessels in the disputed area," US State Department spokesperson Jen Psaki told reporters on Wednesday.

"We call on all parties to conduct themselves in a safe and appropriate manner, exercise restraint, and address competing sovereignty claims peacefully, diplomatically, and in accordance with international law," she added.