A riot at a steel plant in Vietnam left one Chinese worker dead and over 100 injured, as Beijing vowed Thursday it would protect an oil rig in disputed waters at the heart of the unrest.

Long-simmering enmity between the communist neighbours has boiled over in Vietnam with protests in major cities and mobs torching foreign-owned factories after China deployed an oil drilling rig in waters also claimed by Hanoi.

A top Chinese general said his country would continue to man the rig, despite the worst anti-China tensions in Vietnam in decades.

"What we're going to do is ensure the safety of the oil rig and ensure the operation will keep going," General Fang Fenghui, chief of the general staff of the People's Liberation Army, told a news conference after talks at the Pentagon with his US counterpart General Martin Dempsey.

His comments came after worker demonstrations spread to 22 of Vietnam's 63 provinces, according to the Vietnamese government, which called for "tough measures" to bring the escalating situation under control before alarmed foreign investors pull out of the country.

Hundreds of Chinese nationals have fled across the border into neighbouring Cambodia, according to police there, amid fears that a wave of patriotic fervour initially encouraged by Hanoi is getting out of hand.

The unrest is "a very disturbing development and has certainly created the impression that in Vietnam (things) were verging out of control", said Professor Jonathan London at City University of Hong Kong.

Vietnam's communist regime, wary of public gatherings that could threaten its authoritarian rule, has in the past alternated between tolerating anti-China rallies and violently breaking them up.

Experts say Hanoi has allowed some public protests to go ahead recently as a means of expressing extreme discontent with Beijing. The pair have close economic ties but often fraught diplomatic relations.

- 'Indulgence and connivance' -

The deadly riot broke out Wednesday at a steel mill owned by Taiwanese group Formosa Plastics in Vietnam's central Ha Tinh province, following earlier violent protests in the south, where more than a dozen plants were set ablaze and hundreds of protesters detained.

Workers began "attacking some Chinese workers and damaged offices and equipment", Formosa said in a statement.

One Chinese worker was killed and at least 149 people were injured, local official Dang Quoc Khanh said.

China accused Hanoi of acting in concert with the protesters.

The violence in Vietnam had "a direct link with the Vietnamese side's indulgence and connivance in recent days with some domestic anti-China forces and lawbreakers", Beijing's foreign ministry spokeswoman Hua Chunying told reporters.

China lodged "a solemn protest" with Vietnam, the official Xinhua news agency said.

It also said the Chinese Foreign Minister Wang Yi condemned the violence in an urgent phone call with Vietnam's Deputy Prime Minister and Minister of Foreign Affairs Pham Binh Minh, urging Hanoi to take measures to stop the violence.

Earlier, Xinhua said that about 10 Chinese remained unaccounted for after rioters attacked four Chinese companies in Ha Tinh province, citing a Chinese manager.

- History of rivalry -

There is a history of rivalry between China and Vietnam, particularly over the contested Paracel and Spratly islands in the South China Sea.

In 1974, as US troops withdrew from Vietnam, China invaded the Paracel Islands, which were held by the US-backed South Vietnamese regime.

The neighbours fought a brief but bloody border war in 1979 after China invaded following Vietnam's intervention in Cambodia to oust Beijing's Khmer Rouge allies.

They came to blows again in 1988 in the contested Spratly islands in a naval battle in which more than 70 Vietnamese sailors were killed.

However, the two countries normalised relations in 1991 and their economies have become increasingly intertwined.

Beijing's increasing assertiveness in staking its claim to almost all of the South China Sea also has caused concern for other neighbouring countries, particularly the Philippines.

Manila released photographs Thursday to support its claim that China was reclaiming land on a disputed reef in the South China Sea in an apparent effort to build an airstrip.

Tensions between China and Vietnam flared earlier this month after Beijing moved the deep-water drilling rig into waters that Hanoi claims.

There have been repeated skirmishes near the rig in recent days involving vessels from the two countries, with collisions and the use of water cannon.

Prime Minister Nguyen Tan Dung described the situation as "very serious" and said that, while the recent groundswell of patriotism was "the correct thing", instigators who broke the law would be punished.

Export-orientated manufacturing is a key pillar of Vietnam's economy, with high-profile firms -- from electronics giants such as South Korea's Samsung to US sportswear companies like Nike -- producing goods there.

Timeline of Vietnam and China tensions
Hanoi (AFP) May 15, 2014 - A boiling over of tensions between Hanoi and neighbouring giant China has left at least one Chinese worker dead in the worst anti-Beijing riots in decades in Vietnam.

The countries' longstanding territorial disputes over the South China Sea have been peppered by tit-for-tat barbs and occasional clashes.

Key dates over the years:

1974

January: China invades the Paracel Islands held by Washington-backed South Vietnam. Dozens of Vietnamese troops are left dead.

1975

April: Reunification of South and North Vietnam.

1979

February-March: The Sino-Vietnamese Border War breaks out after Hanoi invades neighbouring Cambodia and ousts the Beijing-backed Khmer Rouge. Tens of thousands of lives are lost on both sides.

1988

March: Vietnam and China fight a naval battle on Johnson Reef in the Spratly Islands which killed around 70 Vietnamese.

1991

November: The two countries officially normalise ties

1999

December: A land boundary pact is inked

2000

December: Hanoi and Beijing sign a maritime boundary accord known as the Tonkin Gulf agreement

2011

June: Vietnam holds a live-fire drill in the South China Sea

2012

June: Hanoi passes a law claiming sovereignty over Paracel and Spratly islands, dismissing China's protests as "absurd". At around the same time, Beijing creates the administrative Sansha city in the Paracels, known in China as the Xisha islands.

2012

December: Anti-Beijing protests erupt in Hanoi and Ho Chi Minh City after claims that China had sabotaged a Vietnamese boat. Beginning in 2011, rallies were initially tolerated by Hanoi but later clamped down upon.

2012

November: Vietnamese immigration officers refuse to to stamp entry visas into new Chinese passports featuring a map laying claim to almost all of the South China Sea.

2013

March: China defends taking action against boats that enter its waters, after Hanoi accuses a Chinese vessel of attacking a Vietnamese fishing boat.

2014

May: Beijing deploys a deep-water drilling rig, along with a reported 70 vessels, into contested waters.

May: Hanoi accuses China of colliding with ships and firing water cannon. China in turn accuses Vietnam of ramming vessels.

May: The worst anti-Beijing riots erupt in decades in Vietnam, leaving at least one Chinese worker dead as China accuses Hanoi of acting in concert with protesters.