The United States on Tuesday sharply criticized the movement of a huge Chinese oil rig that Vietnam says has entered its waters.
The Vietnamese statement came days after US 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 its waters in the South China Sea, known in Vietnam as the East Sea, and told China's state-run oil company to remove it.
In Washington, U.S. State Department spokeswoman Jen Psaki told reporters: "Given the recent history of tensions in the South China Sea, China's decision to operate its oil rig in disputed waters is provocative and unhelpful to the maintenance of peace and stability in the region."
"These events point to the need for claimants to clarify their claims in accordance with international law, and reach an agreement ... about what types of activities should be permissible within disputed areas," she added.
Vietnam also protested the move.
"Vietnam cannot accept this, and resolutely protests this action by China," the foreign ministry said on its website, summarizing comments by Foreign Minister and Deputy Prime Minister Pham Binh Minh, who spoke to his Chinese counterpart by telephone on Tuesday.
"We request China pulls out the ... rig and all vessels from this area ... Vietnam will take all suitable and necessary measures to protect our legitimate rights and interests."
A ministry official said the two countries had been in direct talks about the issue since Sunday, but did not say how China had responded to Vietnam's requests. China has claimed 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 wrongfully 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.