China said Wednesday that four people were killed in a riot in Vietnam, doubling the previously announced toll with tensions simmering over a territorial dispute between the two countries.

More than 100 others were injured in the violence last week, said Hong Lei, spokesman for the Ministry of Foreign Affairs.

"The Vietnamese side should learn lessons from that and take concrete actions to ensure the safety of Chinese institutions and personnel and enterprises in Vietnam and safeguard their legitimate rights and interests," he said at a regular briefing.

Beijing has previously said two Chinese nationals were killed when enraged mobs torched or otherwise damaged hundreds of foreign-owned businesses in Vietnam last week.

Hong said the identities of the other two dead were yet to be confirmed.

The announcement came after the state-owned China Metallurgical Group Corporation (MCC) said Tuesday that the unrest had left four of its employees dead and another 126 injured.

Relations between the Communist neighbours have plummeted after the eruption of anti-China rage in Vietnam over Beijing's controversial plans to drill for oil in contested waters in the South China Sea.

According to official media reports, thousands of Chinese have returned from Vietnam since the protests last week.

Hanoi initially lauded "patriotic" displays by its citizens, but backpedalled furiously after the violence -- which hit a number of non-Chinese facilities -- badly stained the country's image as a safe destination for sorely needed foreign investment.

China claims almost all of the South China Sea, parts of which are also claimed by the Philippines, Brunei, Malaysia and Vietnam, all members of the Association of South-East Asian Nations (ASEAN), and non-member Taiwan.

Last week leaders of the 10-nation bloc presented a rare united front by expressing "serious concern" over disputes in the waters, which are home to key shipping lanes and thought to contain vast energy reserves.

Washington has also warned about the potential for tensions to escalate.

Dozens of Chinese and Vietnamese vessels have engaged in repeated skirmishes near the rig, including reported rammings and the use of water cannon.