The Foreign Ministry in Hanoi said the collisions took place on Sunday and caused considerable damage to the Vietnamese ships. Six people suffered minor injuries, it said. (Agencies)
"On May 4, Chinese ships intentionally rammed two Vietnamese Sea Guard vessels," said Tran Duy Hai, a Foreign Ministry official and deputy head of Vietnam's national border committee.
"Chinese ships, with air support, sought to intimidate Vietnamese vessels. Water cannon was used," he told a news conference in Hanoi. Six other ships were also hit, but not as badly, other officials said.
Dozens of navy and coastguard vessels from both countries are in the area where China has deployed the giant rig, Vietnamese officials have said.
"No shots have been fired yet," said a Vietnamese navy official, who could not be identified because he was not authorized to speak to media. "Vietnam won't fire unless China fires first."
The two Communist nations have been trying to put aside border disputes and memories of a brief border war in 1979. Vietnam is usually careful about comments against China, with which it had bilateral trade surpassing $50 billion in 2013.
Still, Hanoi has strongly condemned the operation of the drilling rig in what it says are its waters in the South China Sea, and told the owners, China's state-run oil company CNOOC, to remove it.
The United States has also criticized the move.
The row comes days after U.S. President Barack Obama visited Asia to underline his commitment to allies including Japan and the Philippines, both locked in territorial disputes with China. Obama, promoting a strategic "pivot" towards the Asia-Pacific, also visited South Korea and Malaysia, but not China.
The United States is "strongly concerned about dangerous conduct and intimidation by vessels in the disputed area," U.S. State Department spokeswoman Jen Psaki said in Washington on Wednesday. Psaki reiterated the U.S. view that China's deployment of an oil rig was "provocative and unhelpful" to regional security.
"We call on all parties to conduct themselves in a safe and appropriate manner, exercise restraint, and address competing sovereignty claims peacefully, diplomatically, and in accordance with international law," she told a regular news briefing.
China has not yet responded to the Vietnamese allegations of ramming. Earlier, Foreign Ministry spokeswoman Hua Chunying said the rig's deployment had nothing to do with the United States, or Vietnam.
"The United States has no right to complain about China's activities within the scope of its own sovereignty," she said.
China claims almost the entire South China Sea and rejects rival claims from Vietnam, the Philippines, Taiwan, Malaysia and Brunei.
TENSIONS WITH PHILIPPINES
Tensions are also brewing in another part of the sea, with Beijing demanding that the Philippines release a Chinese fishing boat and its crew seized on Tuesday off Half Moon Shoal in the Spratly Islands.
The boat has 11 crew and police said they found about 350 turtles in the vessel, some already dead. A Philippine boat and its crew was also seized and found to have 70 turtles on board. Several species of turtle are protected under Philippine law.
Police said the boats were being towed to Puerto Princesa town on the island of Palawan where charges would be filed.
China's Foreign Ministry spokeswoman said Hua China had "indisputable sovereignty" over the Spratly Islands and added: "We once again warn the Philippines not to take any provocative actions."
The State Department's Psaki said the United States had seen reports about the boat seizures was concerned that the vessels appeared to be engaged in catching endangered sea turtles.
"We urge both sides to work together diplomatically," she said.
In a commentary, Ernest Bower and Gregory Poling of Washington's Center for Strategic and International Studies think tank called the implications of the rig row "significant."
"The fact that the Chinese moved ahead in placing their rig immediately after President Barack Obama’s visit to four Asian countries in late April underlines Beijing’s commitment to test the resolve of Vietnam, its Association of Southeast Asian Nations neighbors, and Washington," they said.
Beijing may be attempting "to substantially change the status quo" while perceiving Washington to be distracted by developments in Ukraine, Nigeria and Syria, they said.
"If China believes Washington is distracted, in an increasingly insular and isolationist mood, and unwilling to back up relatively strong security assertions made to Japan and the Philippines and repeated during President Obama’s trip, then these developments south of the Paracel Islands could have long-term regional and global consequences," they said.
Tensions are frequent in the South China Sea between China and the other claimant nations, particularly Vietnam and the Philippines, both of which say Beijing has harassed their ships.
However, while there are frequent stand-offs between fishermen and claimant states in the South China Sea, the actual detention of Chinese fishermen or the seizure of a boat is rare.
NOT COMMERCIALLY DRIVEN
An oil industry official in China said deployment of the rig appeared a political decision rather than a commercial one.
"This reflected the will of the central government and is also related to the U.S. strategy on Asia," said the official, who spoke on condition of anonymity.
"It is not commercially driven. It is also not like CNOOC has set a big exploration blueprint for the region."
However, Wu Shicun, president of the National Institute for South China Sea Studies, a Chinese government think tank, said China was unlikely to pay much heed to Vietnamese concerns.
"If we stop our work there as soon as Vietnam shouts, China will not be able to achieve anything in the South China Sea," Wu said.
"We have lost a precious opportunity to drill for oil and gas in the Spratlys. Also this time we are drilling in Xisha (Paracel Islands), not Nansha (Spratlys), there is no territorial dispute there. I think China will keep moving ahead with its plan (in Xisha), no matter what Vietnam says and does."
Tran Duy Hai, the Vietnamese Foreign Ministry official, raised the possibility of Hanoi taking the dispute to international arbitration.
"We cannot exclude any measures, including international legal action, as long as it is peaceful.
"We are a peace-loving nation that has experienced many wars," he said. "If this situation goes too far, we will use all measures in line with international law to protect our territory. We have limitations, but we will stand up to any Chinese aggression."
The Philippines has already taken its dispute with China to an international arbitration tribunal in The Hague.
The Foreign Ministry in Hanoi said the collisions took place on Sunday and caused considerable damage to the Vietnamese ships. Six people suffered minor injuries, it said.