|Hundreds gathered in Vietnam's capital for a rare public protest, demanding that China stay out of their waters [AFP]
Vietnam has started live-fire naval drills in the South China Sea, a move that analysts see as raising the risk of a "showdown" with Beijing over a deepening territorial rift.
"The first live firing by the Vietnamese Navy began at 0100 GMT [on Monday] and will last until 0500GMT," said a naval officer based in the central city of Danang, speaking on condition of anonymity.
The exercise is taking place around Hon Ong island, about 40km off Quang Nam province in central Vietnam, the officer said.
A long-standing dispute between the communist neighbours over sovereignty of two potentially oil-rich archipelagos has erupted again after recent sea confrontations that have left relations at their lowest point in years.
About 300 people in Hanoi, the capital, and Ho Chi Minh City held anti-China rallies on Sunday to proclaim Vietnam's maritime sovereignty.
Demonstrations are rarely allowed in Vietnam, but this was the second weekend in a row that protesters have criticised Beijing.
'Routine annual training'
A second phase of live firing, lasting about six hours, is due to start at 1100 GMT, part of what the foreign ministry has described as "routine annual training".
"I can't reveal the number of Vietnamese ships mobilised for these exercises but there will be gun fire, not missiles," said the officer.
The exercise has raised temperatures in the South China Sea, said David Koh, a Vietnam analyst from the Institute of Southeast Asian Studies in Singapore.
"But I do not think there is much of a choice right now," Koh said, adding that ultimately he foresees "a showdown on the seas".
The drills are inside the area claimed as Vietnam's 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.
The US said it was "troubled" by tensions triggered by the maritime border dispute, calling for a "peaceful resolution".
The live-fire exercise zone is about 250km from the Paracels and almost 1,000km from the Spratlys, the two archipelagos which are claimed by both nations and which straddle strategic shipping lanes.
'Commitment to peace'
Vietnam has made clear its desire for peaceful resolution and adherence to international laws.
Beijing, too, says it is committed to peace in the South China Sea, but its more assertive maritime posture has caused concern among regional nations and beyond.
Tensions have also risen this year between China and the Philippines, another claimant to the Spratlys, where Malaysia, Brunei and Taiwan also say they have a stake.
"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 Pacific Forum CSIS, the Asia-Pacific arm of the Centre for Strategic and International Studies in Washington.
Despite that possibility, Cossa said all sides would ensure that any escalation would "not get out of control".
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.
In the same area, in July 2007, China's navy reportedly fired at a Vietnamese fishing boat, killing one sailor.