Vietnamese prime minister Nguyen Tan Dung said his government is weighing a number of defence options after China deployed an oil rig in disputed waters in the South China Sea, according to an exclusive report from Reuters.

In a written statement sent to the news agency, Dung revealed for the first time that Hanoi was considering legal measures against Beijing.

"Vietnam is considering various defence options, including legal actions in accordance with international law," Dung said in the statement on Wednesday. "I wish to underscore that Vietnam will resolutely defend its sovereignty and legitimate interests because territorial sovereignty, including sovereignty of its maritime zones and islands, is sacred."

The move is likely to incense Beijing, which has refused to participate in a separate case brought to an arbitration tribunal in The Hague by the Philippines in March.

The case marks the first time China has been brought before an international court over its territorial claims to the South China Sea.

Beijing claims almost all of the sea for itself, overlapping with other countries' claims to the potentially energy-rich waters.

Anti-China riots erupted across Vietnam last week after a Chinese oil rig was deployed in waters close to the Paracel islands, an area also claimed by Vietnam. Thousands attacked businesses and factories across Vietnam, leaving four people dead.

Vietnam said the Chinese rig is parked within its 200-nautical mile exclusive economic zone, while China said the rig was operating entirely within its waters.

The countries' navies clashed near to the islands two weeks ago, both sides accusing the other of ramming the others' ships and firing water cannons.

The United States responded to the skirmishes with a message of support for its ally Vietnam, describing China's actions as "provocative."

US Secretary of State spoke with his Vietnamese counterpart Pham Binh Minh on Wednesday, during which Kerry invited Minh to visit Washington.

While China claims around 90% of the South China Sea, the Philippines, Malaysia, Brunei, and Taiwan also have claims to parts.