Beijing: Chinese and Vietnamese coastguard vessels have been involved in a week-long confrontation over a reef in the South China Sea, according to a media report which said the development could cause the biggest clash between the two neighbours in five years.

Six heavily armed coastguards vessels - two Chinese and four Vietnamese - have been eyeing each other in patrols around the Vanguard Bank in the Spratly group of islands since last week. About a dozen vessels were reported in the vicinity, the Hong Kong-based South China Morning Post reported.

On Wednesday last week, Chinese survey ship Haiyang Dizhi 8 entered waters near the Vietnamese-controlled reef to conduct a seismic survey, Ryan Martinson, an assistant professor at the US Naval War College in Newport, Rhode Island, said in a tweet on Friday, citing ship tracking data.

Its escorts included the 12,000-tonne armed coastguard vessel 3901, complete with helicopter, and the 2,200-tonne coastguard ship 37111.

The standoff came despite a pledge in May by Chinese and Vietnamese defence ministers to settle maritime disputes by negotiation.

According to the report, the standoff may trigger a wave of anti-China sentiment in Vietnam not seen since 2014, when a Chinese oil rig arrived off the disputed Paracel Islands.

China claims sovereignty over almost all of the 3.56 million sq kms of the South China Sea (SCS) which is disputed by Vietnam, the Philippines, Malaysia, Brunei and Taiwan.

Vietnam fiercely asserts its rights over a section of the SCS.

An attempt by China in 2014 to drill oil in Paracel islands claimed by Hanoi had led to anti-China riots in Vietnam in which several Chinese factories have been vandalised.

The Chinese Foreign Ministry spokesman Geng Shuang Friday did not confirm the standoff when asked about the repot, but asserted China's claims over the area.

"China position in the SCS is clear consistent. We firmly uphold our interests and sovereignty rights in the South China Sea," he said.

"But we are also committed to managing our differences through negotiations with relevant countries," Geng said.

Chinese President Xi Jinping meanwhile told visiting Vietnamese National Assembly chairwoman Nguyen Thi Kim Ngan that the two countries should "safeguard maritime peace and stability with concrete actions".