Hanoi said Wednesday that Chinese ships protecting a deep-water drilling rig in disputed waters in the South China Sea had used water cannon to attack Vietnamese patrol vessels and repeatedly rammed them, injuring six people.

Tensions between the communist neighbours have risen sharply since Beijing unilaterally announced last week it would move the deep-water drilling rig into disputed waters -- a move the United States has described as "provocative".

Vietnam deployed patrol vessels after the China Maritime Safety Administration issued the unilateral navigational warning on its website saying it would be drilling in the South China Sea close to the Paracel Islands -- which are controlled by China but claimed by Vietnam.

Vietnam said China's decision was "illegal", demanded the rig be withdrawn, and dispatched vessels to the area.

Ngo Ngoc Thu, deputy commander of Vietnam's maritime police, told reporters in Hanoi Wednesday that Chinese boats had collided with Vietnamese vessels in at least three separate incidents since the May 3 announcement.

A Chinese plane had also flown low over Vietnamese police patrol boats dispatched to the area in a bid to threaten them.

He said the Chinese "actively used water cannon to attack Vietnamese law enforcement vessels".

"The situation was very tense," he said, adding that "six Vietnamese fisheries surveillance staff were injured due to broken glass".

"We broadcast a signal asking the rig to leave the area. We have showed that we are patient and self-restrained in the face of Chinese aggressive acts," he said, adding that Vietnam had not dispatched military ships to the area -- only police and coastguard patrol boats.

"Our patience is limited. If they (China) continue to hit us, we will have to take self-defence measures in return," he warned.

Vietnam broadcast a video of the alleged ramming incidents.

China claims sovereign rights to almost the whole of the South China Sea, which is believed to sit atop vast oil and gas deposits.

- 'Disruptive activities' -

Chinese foreign ministry spokeswoman Hua Chunying reiterated Beijing's position that the rig was in Chinese territory.

"The drilling activity of this rig is within China's territorial waters. The disruptive activities by the Vietnamese side are in violation of China's sovereign rights," she said.

"The drilling activities on the rig are completely legal, and we ask the Vietnamese side to stop their disruptive actions."

The South China Sea is also claimed in part by Taiwan, Brunei, Malaysia and the Philippines.

Elsewhere in the South China Sea, the Philippines said it had seized a Chinese fishing vessel in disputed waters and detained its 11 crew members, in the latest escalation of their own bitter row.

Officials said the 15-tonne Chinese boat had been intercepted Tuesday while fishing off Half Moon Shoal, west of the major Philippine island of Palawan, in what they said were Philippine waters.

"The seizing of the Chinese fishing boat... (was) undertaken as actions to enforce maritime laws and to uphold Philippine sovereign rights over its Exclusive Economic Zone," a Filipino foreign department statement said.

Beijing called for Manila to release the vessel and the crew.

"We ask the Philippines side to give their explanation and deal with this case properly," foreign ministry spokeswoman Hua said.

China and Vietnam, which fought a brief border war in 1979, have been locked in a longstanding territorial dispute over their contested waters, and frequently trade diplomatic barbs over oil exploration, fishing rights and the Spratly and Paracel Islands.

Vietnam's authoritarian rulers have also been struggling to control intense domestic criticism of their handling of relations with China.

Some calls have already gone out on dissident websites for an anti-China protest in Hanoi -- previous protests have been broken up by police and resulted in arrests.

Vietnam expert Carl Thayer said the decision to move the oil rig into disputed waters was "a major change in China's strategy", adding it could be a response to US President Barack Obama's recent Asia tour.

During the tour Obama asserted US support for allies Japan and the Philippines, which are also locked in territorial disputes with Beijing.

US State Department spokeswoman Jen Psaki said Washington was "strongly concerned about dangerous conduct and intimidation by vessels in the disputed area", calling for restraint in what Vietnam calls the East Sea.

Speaking about the seizure of the Chinese fishing boat by the Philippines, Psaki urged both sides to "work together diplomatically".