Vietnam put on a show of military strength in the tense South China Sea on Monday, risking the ire of Beijing in the face of a deepening maritime rift with its powerful neighbour.

Relations between the communist nations have sunk to their lowest point in years following recent sea confrontations which reignited a long-standing dispute over sovereignty of two potentially oil-rich archipelagos.

A successful first barrage of naval artillery, lasting about four hours, took place about 40 kilometres (25 miles) off Quang Nam province in central Vietnam, said a naval officer in Danang city who asked not to be named.

He declined to reveal how many ships had been mobilised but said no missiles were fired. A similar night drill started at 7:00 pm (1200 GMT) and lasted about five hours, the officer added.

AFP's request to witness the exercise was not granted.

Although Vietnam's foreign ministry described it as routine annual training, analysts say the drill has raised temperatures in the South China Sea, where Asian nations have conflicting claims over possibly energy-rich waters.

The exercise "is designed to send China a message that Vietnam refuses to be pushed around", said Ian Storey, a regional security analyst with the Institute of Southeast Asian Studies in Singapore.

"I think the Chinese will react very badly to this," he added.

The drills are inside the area Vietnam claims as its 200-nautical-mile economic zone.

Hanoi last month accused Chinese surveillance vessels of cutting the exploration cables of an oil survey ship inside the area.

On Thursday Vietnam alleged a similar incident in the zone, saying a Chinese fishing boat rammed the cables of another oil survey ship in a "premeditated" attack.

Beijing countered by warning Vietnam to halt all activities that it says violate China's sovereignty in the disputed area.

"No one wants a war but the possibility of some shots being fired in anger or of some ships running into other ships has increased," said Ralph Cossa, president of the Hawaii-based Pacific Forum CSIS, a research institute.

The United States says it is "troubled" by the tensions triggered by the maritime dispute.

But US senator Jim Webb, who heads the Senate Foreign Relations subcommittee on East Asia, urged Congress to do more and condemn China over the growing number of maritime rifts.

"I think we in our government have taken too weak of a position on this," said Webb, a member of President Barack Obama's Democratic Party.

The naval drill was about 250 kilometres from the Paracel Islands and almost 1,000 kilometres from the Spratlys, the archipelagos which are claimed by both nations and which straddle strategic shipping lanes.

Vietnam has said it wants to see a peaceful resolution and adherence to international law.

Beijing, too, says it is committed to peace in the South China Sea, but its more assertive maritime posture has caused concern regionally and beyond.

Tensions have also risen this year between China and the Philippines, another claimant to the Spratlys, which on Monday said it would from now on refer to the South China Sea as the "West Philippine Sea".

Taiwan on the weekend reiterated its claim to the Spratlys, and said missile boats and tanks could be deployed to disputed territory.

Brunei and Malaysia have also staked claims in the area.

Vietnamese bitterly recall 1,000 years of Chinese occupation and, more recently, a 1979 border war. More than 70 Vietnamese sailors were killed in 1988 when the two sides battled off the Spratlys.

About 300 people in Ho Chi Minh City and in Hanoi held anti-China rallies on Sunday to proclaim Vietnam's maritime sovereignty for the second weekend in a row. Demonstrations are rarely allowed in Vietnam.

In interviews, protesters voiced support for the naval drill. "It shows to China and to the world that we will do everything to protect our land and our sea," said Tran Bao, 36.