Thousands of Vietnamese set fire to factories and rampaged in industrial zones in the south of the country after protests against Chinese oil drilling in a part of the South China Sea claimed by Vietnam.
The brunt appears to have been borne by Taiwanese companies in the zones in Binh Duong and Dong Nai provinces as rioters mistook the firms to be Chinese-owned.
Vietnamese officials gave few details, but said gates to factories were smashed and windows were broken. Police said they were investigating.
A Singapore foreign ministry spokesman said the premises of a number of foreign companies were broken into and set on fire in the Vietnam-Singapore Industrial Parks (VSIP) I and II in Binh Duong.
The spokesman said the Singapore government had asked Vietnam to immediately restore law and order, but gave no other details.
"About 19,000 workers were demonstrating against China's violation of Vietnam's territorial waters," Tran Van Nam, vice chairman of the Binh Duong People's Committee, told local reporters in the province.
"Some workers turned angry, destroying companies' gates and entering the compounds and asking other workers to join a strike."
Taiwan's Formosa Plastics Group said property was damaged by looters, but did not give details. There were no reports of injuries.
"Everyone is terrified and scared," said Serena Liu, chairwoman of the Taiwan Chamber of Commerce in Vietnam.
"Some people tried to drive out of Binh Duong, but looters had put up road blocks."
Hong Kong-listed sports shoe maker Yue Yuen, which supplies footwear to Adidas, Nike and other international brands, said it had suspended production in Vietnam because of the protests.
China has urged Vietnam to "calm down" and respect China's sovereignty, Foreign Minister Wang Yi said in comments to Indonesia's Foreign Minister Marty Natalegawa in a telephone call, Beijing's foreign ministry reported on its website.
Anti-China sentiment also surged in Manila, as the Philippine government accused Beijing of reclaiming land on a reef in disputed islands in another part of sea, apparently to build an airstrip. The action came only a day after Washington described Beijing's actions in the region as "provocative".
"If these reports are true, this would represent a significant step by the Chinese, potentially allowing them to extend their airborne reach," said Ian Storey, a South China Sea expert at Singapore's Institute of South East Asian Studies.
The spike in tensions over the oil- and gas-rich South China Sea comes just two weeks after U.S. President Barack Obama visited the region and expressed support for long-time allies Japan and the Philippines, both of which are locked in territorial disputes with China. Vietnam is also stepping up ties with the United States.
China claims almost the entire South China Sea, an area rich in energy deposits and an important passageway traversed each year by $5 trillion worth of ship-borne goods.
Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims on the area.
AIR DEFENCE ZONE?
Philippine foreign affairs department spokesman Charles Jose told Reuters that China had been moving earth and materials to Johnson South Reef, known by the Chinese as Chigua and which the Philippines calls Mabini Reef, in recent weeks.
He said China was reclaiming land in violation of the Declaration on the Conduct of Parties in the South China Sea, an informal code of conduct for the region.
"I think they're to construct an airstrip there," Jose said.
Peter Paul Galvez, a defence department spokesman, said evidence of the Chinese activity on the reef had been shown in aerial photographs taken by the Philippine Navy. The Philippines and Taiwan already have airstrips in the area.
A senior Philippine air force official told Reuters that a landing strip in the area would give China better control of the air and sea lanes in the South China Sea.
"It will be easy for them to set up an air defence identification zone in the Spratlys," he said, speaking on condition of anonymity.
However, Richard Bitzinger, a military analyst at the S. Rajaratnam School of International Studies in Singapore, said the airstrip was unlikely to be a strategic game-changer in the South China Sea because of the difficulty in building a workable runway on the atoll.
"It would be a nice tool to have in the box of options to project power, but it is probably going to be far too small to have a huge impact," Bitzinger said.
"At this point I would be very surprised to see this develop into any airbase of any significant size...China's holdings in the Spratlys are just too small.
"It is probably as much a political move as anything else, the laying down of one more marker to solidify their position and continue their campaign of creeping assertiveness."
Manila had already lodged a protest with the Chinese and raised the issue behind closed doors at last weekend's summit of the Association of South East Asian Nations in Myanmar, Jose said.
Tensions in the South China Sea were already high after China moved a large oil rig into an area claimed by Vietnam. Beijing and Hanoi each accused the other of ramming its ships near the disputed Paracel Islands.
On Tuesday, U.S. Secretary of State John Kerry said during a phone call with Chinese Foreign Minister Wang Yi that China's introduction of the oil rig and numerous government vessels into the area disputed with Vietnam was "provocative", a State Department spokeswoman said.
China said Kerry said no such thing in the phone call.
It said there had indeed been provocative action taken in the area but that it was not the guilty party, with the foreign ministry blaming the United States for encouraging such behaviour.
The ministry said Wang had urged Kerry to "act and speak cautiously".
Beijing says the South China Sea issue should be resolved by direct talks between those involved and has bristled at what it sees as unwarranted U.S. interference.
It has also looked askance at the U.S. "pivot" back to Asia, especially Washington's efforts to boost existing military links with Tokyo and Manila.
The remote and otherwise unremarkable Johnson South Reef has been a catalyst for conflict in the past. In March 1988, China and Vietnam fought a brief naval skirmish on and around the reef with up to 90 Vietnamese reported killed.