Chinese ships purposely collided with two Vietnamese vessels in disputed waters where Chinese firms started operating at the weekend, causing large scale damage, Vietnam's foreign ministry said on Thursday.
China claims almost the entire oil- and gas-rich South China Sea, rejecting rival claims to parts of it from Vietnam, the Philippines, Taiwan, Malaysia and Brunei. It also has a separate maritime dispute with Japan.
"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.
Hai said Vietnam had requested talks with high-level leadership in Beijing and was awaiting a response.
Other countries in the region were concerned, he said, adding that Vietnam would do "everything necessary" to settle the dispute peacefully.
Reporting by Nguyen Phuong Linh