China and Vietnam have been locked in a dispute at Vanguard Bank in the South China Sea.
The stand-off began when the Chinese ship Haiyang Dizhi 8 entered the waters near the reef to ostensibly conduct a seismic survey, days after Vietnam entered into a joint exploration agreement with Japan in the reef area.
Earlier, China had hinted that it might decide to drill in the area if Vietnam does not back off.
The situation became fraught after Vietnam announced on July 25 it would allow the Japanese exploratory oil rig to continue operations beyond the earlier planned completion date of July 30. Before that Beijing had asked Vietnam to withdraw the rig in exchange for China withdrawing the survey ship. In response, an angry Vietnam has decided to deploy more patrol ships in the area.
The confrontation between the two nations is the biggest since 2014, when a Chinese oil rig arrived off the disputed Paracel Islands, leading to a skirmish.
In the current stand-off, heavily armed coastguard vessels of both nations have been in a confrontationist situation around Vanguard Bank in the Spratly group of islands.
This time, Vietnam appears to have decided to dig in its heels - in departure from its earlier stance when it would respond to Chinese threats by backing off.
Last week, Vietnam asked China to cease its unlicensed seismic exploration activity within the Vietnamese Exclusive Economic Zone.
On July 17, Chinese foreign ministry spokesman Geng Shuang reiterated China's sweeping claim to sovereignty over the majority of the South China Sea.
According to reports, the Chinese ship has been active in an area to the northeast of Vietnam's Block 06-01 gas field, that supplies about 10 per cent of all of Hanoi's energy consumption.
On Saturday, the US expressed concern over China hampering oil and gas activities by other nations in the South China Sea and asked Beijing to stop acting like a bully.
The South China Sea is one of the world's busiest waterways. China claims more than 80 per cent of the waters, while Vietnam claims sovereignty over the Paracel Islands and the Spratlys.
According to reports, Vietnam has briefed India about the escalating tension in the sea.
Hanoi has always been keen for India to play a more active role in maintaining peace and stability in the region.
The South China Sea is a resource-rich strategic waterway through which more than $5 trillion worth of world trade is shipped each year.
Vietnam maintains that parties should settle the dispute in the sea through peaceful means in line with international law including UNCLOS (United Nations Convention on the Law of the Sea).
The latest stand-off occurs despite a pledge in May by Chinese and Vietnamese defence ministers to settle maritime disputes through negotiation.
( With inputs from IANS )