China's coastguard has detained six Vietnamese fishermen, an official said Friday, in a move that could raise tensions between the neighbouring countries, already locked in a bitter maritime dispute.

The seizure of the fishing vessel and its crew on Thursday was witnessed by other Vietnamese sailors in the area, an official in central Quang Ngai province told AFP on condition of anonymity.

The state-run Tuoi Tre newspaper said the six men were detained by a Chinese fisheries surveillance vessel in "common fishing grounds" in the Tonkin Gulf in the disputed South China Sea.

But Chinese foreign ministry spokesman Hong Lei said the Vietnamese fishing vessel was seized "for illegal operation, within Chinese territorial waters," some seven nautical miles south of Hainan island.

"The relevant Chinese departments are investigating the case in accordance with the law," he added.

China has captured Vietnamese fishermen before but this is the first such incident since China moved a deep sea oil rig into contested waters near the Paracel Islands in May.

Relations between the communist neighbours have plummeted over the oil rig's presence, which has worsened an increasingly heated dispute over territorial claims in the South China Sea.

Vietnamese foreign ministry spokesman Le Hai Binh told reporters on Thursday that Hanoi was "carefully considering" possible legal action against Beijing over the deployment of the oil rig.

The ministry did not immediately respond to requests for comment Friday.

The standoff has seen repeated skirmishes between dozens of Chinese and Vietnamese vessels, including many civilian and fishing boats.

The confrontations have included reported rammings and the use of water cannon. Vietnam has accused China of ramming and sinking one of its fishing boats.

Tensions over the oil rig sparked violent anti-Chinese riots in Vietnam in May. Beijing says four Chinese citizens died in the unrest, while Vietnam put the toll at three.

China claims nearly all of the South China Sea, even waters approaching the coasts of its neighbours, and has become increasingly assertive in staking those claims.

Vietnam, the Philippines, Brunei and Malaysia, as well as Taiwan, have competing claims to parts of the sea, which sits on key shipping lanes and is believed to harbour huge oil and gas deposits.

In recent years China has begun aggressively patrolling contested waters, using fishing bans and patrol boats to keep foreign trawlers out, according to Vietnamese officials and fishermen.

Hanoi says hundreds of fishing boat crews have been arrested by Chinese authorities over the past few years.