The war of words between China and Vietnam over disputed islands in the South China Sea has gained steam and, while never dormant, the heated rhetoric has become notably more direct.

As the 14th ASEAN (Association of Southeast Asian Nations) summit was in progress last weekend in Naypydaw, Myanmar's capital, thousands of Vietnamese in the capital Hanoi and in Ho Chi Minh City took to the streets in anger after the China National Offshore Oil Corporation moved drilling rig HD-981 near the contested Paracel Islands.

Tensions further increased when Vietnamese naval ships, trying to prevent the Chinese from setting up the oil rig, collided with Chinese navy vessels. Beijing and Hanoi each accused the other of ramming their ships.

Occupied by China since 1974, sovereignty over the area surrounding the Paracel's has been challenged by China, Taiwan and Vietnam ever since.

Vietnamese anger has now erupted into deadly violence in the country's south. More than 20 people were killed on Wednesday after rioters stormed a foreign-owned steel factory in Ha Tinh province, with 16 of the dead said to be Chinese. About 100 other people were sent to hospital with injuries.

Thousands of Vietnamese set fire to foreign factories and rampaged through industrial zones in Binh Duong and Dong Nai provinces on Tuesday. News reports said hundreds of Chinese had fled the country.

We can read the Chinese [oil rig] move was also a response to Obama's pivot to Asia-Pacific. - Hoang Viet, Ho Chi Minh City University

US 'pivot to Asia-Pacific'

Hoang Viet, a professor at Ho Chi Minh City University, noted the rig incident came days after an official visit to Asia by US President Barack Obama, who reiterated his commitment to allies in the region, including Japan, the Philippines, South Korea and Malaysia.

"Hence, we can read the Chinese move was also a response to Obama's pivot to Asia-Pacific," Viet told Al Jazeera.

In Manila, President Benigno S Aquino III and Obama recently signed a 10-year pact that will give the US military greater access to the Philippines.

"The United States is not trying to reclaim old bases or build new bases… We'll work together to build the Philippines' defence capabilities and to work with other nations to promote regional stability, such as in the South China Sea," Obama said during the April visit.

Tensions among China and other nations surrounding the South China Sea have risen sharply recently.

The Philippine government seized a Chinese fishing boat and its 11 crew earlier this month on charges of catching endangered sea turtles in disputed South China Sea waters, prompting China to demand their release. China foreign ministry spokeswoman Hua Chunying urged the Philippine government to "stop taking further provocative actions".

The Philippines accused China on Wednesday of building an airstrip on a reef in the South China Sea in disputed territory. If proven true, it would be the first airstrip constructed by China on the contested Spratly Islands, and would violate the Declaration on the Conduct of Parties in the South China Sea, an informal code of conduct for the region.

Complex issues

Viet said when it comes to China-Vietnamese relations, there's more than meets the eye in the latest flare-up over the disputed territory. "There are also other factors that are raising tensions, and the issue is more complex that it appears," he said.

Over the past decade, both sides have repeatedly stated their intention to achieve a peaceful solution through negotiations, but at present there is little or no direct dialogue between the two governments.

Yet, the two communist countries are bound by significant economic ties, with bilateral trade expected to reach $60bn in 2015, growing from $50bn in 2013. Vietnam's trade deficit with China in 2013 stood at $23.7bn, an increase of 44.5 percent compared to the previous year, according to Vietnam's customs office.

"China and Vietnam need each other. The Vietnamese economy cannot exist without the Chinese one," Pietro Masina, a professor of international relations and economics at Napoli l'Orientale University in Italy, told Al Jazeera.

"There is a sort of paradox with a conflict on one hand - that I would say is regional and not only between China and Vietnam - and strong economic integration on the other hand."

Analysts have underlined the necessity for Hanoi and Beijing to negotiate over the contested areas in the South China Sea - one of the world's most important transit routes and fishing areas, which is also believed to be rich in fossil fuels. However, any potential cooperation can only reach fruition amid common ground found between Vietnam and China, as well as countries equally involved including Brunei, Malaysia, and Taiwan.

Vietnam insists on the necessity to work at multilateral level and within the ASEAN framework. However, China - which claims almost the entire South China Sea as its own - has made clear its ready to negotiate bilaterally only.

'Serious concerns'

During the recent ASEAN summit, the grouping's 10 foreign affairs ministers expressed "serious concerns" over the recent developments in the South China Sea.

China's foreign minister spokeswoman Hua responded to the criticism by saying the South China Sea dispute is not a problem between ASEAN and Beijing.

"China is opposed to certain countries' scheme of spoiling the atmosphere of friendly cooperation between China and ASEAN by making use of the issue of the South China Sea," Hua said. "China is ready to press ahead with the comprehensive and effective implementation of the Declaration on the Conduct of Parties in the South China Sea [DOC] together with ASEAN countries, so as to safeguard peace and stability of the region."

International security analyst Rommel Banlaoi told Al Jazeera it was unusual for ASEAN to speak out against China, even though it did not directly cite Beijing's oil rig deployment specifically.

"ASEAN is a diplomatic community of sovereign states with different levels of relationship with China," Banlaoi said. "ASEAN is cautious in using strong words in order not to antagonise China. But when ASEAN uses the words 'serious concerns', China knows that the problem is getting worse, and it is a signal to China to make up with its neighbours."

In the past we fought hard to regain independence for the people, and thus we cannot accept the escalation of Chinese actions. - Nguyen Thi Huan, protester

Professor Masina, who has worked in both China and Vietnam, said Beijing's oil-rig deployment may backfire in its long-term efforts of securing the resource-rich region for itself.

"In some way, the Chinese aggressive behaviour in the contested area is a sign of weakness," Masina said. "A powerful country that wants to be a regional power is one which builds consensus around its policies. And if China is able to stand out by military force, politically it is weak. It is in the Chinese interests to find a negotiable solution to the issue."

Nationalism on the rise

While politicians attempt to find a way out of the impasse, nationalistic sentiment is rising both in China and Vietnam.

"When the enemy is at our doors, women also have to fight," said Nguyen Thi Huan, who battled the Americans during the Vietnam War. She arrived in Hanoi from Vinh Phuc province last Sunday to demonstrate against China.

"In the past we fought hard to regain independence for the people, and thus we cannot accept the escalation of Chinese actions," Huan told Al Jazeera.

Professor Viet said the rise of nationalistic fervour in both China and Vietnam over South China Sea possessions bodes ill for both countries.

"Tension at the moment is very high and both sides - who see each other as a provocative opponent - are trying to conquer public opinion… Nationalist sentiment is increasing and in international politics, it does not favour any of the parties," Viet said.