Vietnam has accused China of attacking its ships in the South China Sea, waters that have become a source of friction between the regional giant and its neighbours.
The Foreign Ministry in Hanoi said on Wednesday that the collisions caused considerable damage to the Vietnamese ships, Reuters news agency reported.
"On May 4, Chinese ships intentionally rammed two Vietnamese Sea Guard vessels," said Tran Duy Hai, a Foreign Ministry official.
"Chinese ships, with air support, sought to intimidate Vietnamese vessels. Water cannon was used," he told a news conference in Hanoi. Six people suffered minor injuries as a result of the incident.
China has yet to respond to the allegations. The state-run oil company, CNOOC, is operating a drilling rig in waters claimed by Hanoi.
Jen Psaki, spokesperson for the US State Department, said on Wednesday: "Given the recent history of tensions in the South China Sea, China’s unilateral decision to introduce its oil rig into these disputed waters is provocative and unhelpful to the maintenance of peace and stability in the region.
"We are strongly concerned about dangerous conduct and intimidation by vessels in the disputed area."
Hanoi's complaint follows an incident between China and the Philippines, after the Philippine government seized a Chinese fishing boat and crew on charges of catching endangered sea turtles in disputed South China Sea waters, prompting China to demand their release.