The Vietnamese accusation came days after U.S. President Barack Obama visited Asia to underline his commitment to allies there, including Japan and the Philippines who are themselves locked in territorial disputes with China.

Obama, promoting a strategic "pivot" toward the Asia-Pacific region, also visited South Korea and Malaysia, but not China.

Vietnam has condemned the operation of the deepwater drilling rig in what it says are its waters in the East Sea and told China's state-run oil company to remove it. China said the rig was operating completely within its waters.

Daniel Russel, Assistant U.S. Secretary of State for East Asian and Pacific Affairs, said the United States was looking into the matter, but urged caution from all sides.

"We believe that it is critically important for each of the claimant countries to exercise care and restraint," he told Reuters during a visit to Hong Kong ahead of a previously scheduled trip to Hanoi on Wednesday.

"The global economy is too fragile and regional stability is too important to be put at risk over short term economic advantage."

China claims almost the entire oil- and gas-rich East 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.

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Its claims coincide with growing diplomatic and military influence in the region and have raised fears of possible conflict.

The Maritime Safety Administration of China (MSAC) announced on its website on Saturday that all vessels should keep one mile away from the rig, called the Haiyang Shiyou 981. It expanded that to three miles on Monday.

The $1 billion rig is owned by China's state-run CNOOC oil company and it had been drilling south of Hong Kong.

On Sunday, Vietnam's Foreign Ministry spokesman objected to the Chinese announcement, saying the coordinates of the oil rig put it in Vietnam's exclusive economic zone and on its continental shelf, about 120 nautical miles off its coast.

The spokesman, Le Hai Binh, said in a statement Vietnam "resolutely opposed" the Chinese company's drilling.