Vietnam and China have plunged into a new war of words over Asia's most hotly contested pieces of real estate, the Spratly Islands.
For the second week in a row, hundreds of Vietnamese nationalists have been holding rare public demonstrations outside the Chinese embassy in Hanoi and the Chinese consulate in Ho Chi Minh City.
Shouting anti-Chinese slogans and singing patriotic songs, they accuse China of staging a creeping invasion of the Spratlys, which have become one of Asia's major potential flashpoints.
Most of the islands are low-lying coral reefs and rocky outcrops in the middle of the South China Sea, home to little more than a few dozen seabirds. Some of them are so small they are covered at high tide.
Yet the island chain is strategically located in the centre of one of Asia's largest potential reservoirs for oil and natural gas, and surrounded by rich fishing grounds.
Six nations, including China, have staked overlapping claims to the 200 islands, rocks and reefs that make up the chain.
Vietnam, Malaysia, the Philippines and Brunei have claims to some of the islands, while China claims sovereignty over them all. Taiwan's claim is similar to China's.
There have been numerous military skirmishes in the past 30 years to reinforce the conflicting claims, the most serious in 1976, when China invaded and captured a nearby island chain, the Paracel Islands, from Vietnam.
Twelve years later, the two countries clashed again as their navies waged a brief battle off Johnson Reef in the Spratlys. Several Vietnamese boats were sunk and more than 70 sailors died.
Since then, Beijing and Hanoi have tried to ease tensions by promising to seek a diplomatic solution.
But China has continued to build military installations on some of the islands and reefs, insisting they are only shelters for Chinese fishermen.
More recently, the legislature in Beijing ratified a plan to manage the Paracels and Spratlys as a new administrative district of Hainan province, turning the islands into a new "county-level city" called Sansha.
That has infuriated Vietnam, which tried last spring to let drilling and pipeline rights for a US$2-billion gas field to energy giant BP in an area of the Spratlys off its southern coast.
When Beijing accused Hanoi of infringing Chinese territory, the company decided to halt exploration work.
Still, Vietnam insists many of the Spratly Islands lie within the bounds of its sovereignty and it resents China's claims, which are backed by an assertive new nationalism and one of its biggest military spending sprees ever.
Regional rivalries take on an added geopolitical importance because the islands straddle Asia's most vital seal lanes.
About 25% of world shipping passes through the region, carrying Middle East oil to Japan and the western United States.
Washington's alliances and defence agreements with countries in the region could drag the United States into a confrontation with China if the conflict over the Spratlys turns violent.
That concerns Washington, because in 1995 the U.S. Naval War College ran a series of computer war games simulating a conflict with China over the South China Sea, and in each case China won.
Since then, Beijing has spent billions modernizing and expanding its navy with an eye to a possible confrontation in the Spratlys.
China has filled a virtual power vacuum in the South China Sea after the end of the Cold War and the withdrawal of the former Soviet Union's navy from Vietnam's Cam Ranh Bay and the U.S. withdrawal from Subic Bay in the Philippines.
As if to assert that fact, China infuriated Vietnam by staging a naval exercise in the South China Sea in November near the Paracels.
Now, Beijing is accusing Vietnam of threatening relations between the two countries by permitting street demonstrations in front of the Chinese embassy for two weekends in a row.
The Vietnamese Foreign Ministry insists the protests were spontaneous and quickly ended by police.
But a Chinese Foreign Ministry spokesman said, "We are highly concerned over the matter. We hope the Vietnamese government will take a responsible attitude and effective measures to stop this and prevent bilateral ties from being hurt."