Vietnam said Wednesday that a Chinese ship intentionally rammed two of its vessels in an area of the disputed South China Sea where Beijing has deployed a giant oil rig, spouting tensions in the region, Reuters reported.

The collisions occurred Sunday and caused significant damage to the Vietnamese ships, Hanoi's Foreign Ministry said.

According to the Guardian, the Chinese ships have also used water cannons at the Vietnamese vessels. Several boats have been damaged and at least six Vietnamese on board have been injured, officials said.

"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 deployed the rig, Vietnamese officials 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 attempting to push aside border disputes as well as memories of a short border war in 1979, Reuters said.

Vietnam is often careful about making comments against China. In 2013, Vietnam had bilateral trade with China that surpassed $50 billion.

Even so, Hanoi has strongly condemned the operation of the drilling rig in what it claims 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, Reuters reported.

These events come days after President Obama's visit in Asia to punctuate his commitment to allies including Japan and the Philippines, which are both caught in territorial disputes with China, which Obama didn't visit.

On Wednesday, U.S. State Department spokeswoman Jen Psaki said that the U.S. is "strongly concerned about dangerous conduct and intimidation by vessels in the disputed area."

Psakis reiterated the United States' view that China's deployment of an oil rig was "provocative and unhelpful" to security in the region.

"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," Psakis told a news briefing.

China hasn't responded to Vietnam's 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.