Vietnam's president has voiced firm opposition to China's claims in the South China Sea, but declined to back a bid by the Philippines to take the row to the UN.
On a visit to Washington, President Truong Tan Sang rejected China's "nine-dash line" through which it claims virtually all of the strategic sea, including islands close to neighbours.
"We cannot find any legal foundation or scientific basis for such a claim and therefore it is the consistent policy of Vietnam to oppose the nine-dash line plan by China," Sang told the Centre for Strategic and International Studies.
But he declined to comment when asked if Vietnam would join the Philippines which in January said it was asking an arbitration panel of the UN Convention on the Law of the Sea to declare China's claims invalid.
"As a member of the United Nations, the Philippines has the legal right to carry on with any proceedings they would like," Sang said.
The Philippines and Vietnam have led criticism of what they consider increasingly assertive claims by China in the South China Sea.
The Philippines has had especially tense relations with China, which seized the Scarborough Shoal, an outcrop claimed by Manila, after a two-month naval stand-off last year.
But friction has eased slightly between Vietnam and China, with Sang visiting Beijing last month and agreeing to set up a hotline to try to prevent mishaps from escalating.
China separately has increasingly butted heads with Japan, which fears that Beijing is trying to exert control over resource-rich waters in the East China Sea.
Sang earlier met US President Barack Obama, who encouraged calm in the South China Sea.
Sang and Obama called for "the settlement of disputes by peaceful means" and renewed support for a code of conduct to manage potential mishaps.
It was the first visit of a Vietnamese leader to the White House during the Obama administration, and it took on an even greater strategic resonance, given Obama's determination to increase the US presence in Asia.
They pledged to deepen trade and military ties, even as they tangled over human rights.
Sang's visit follows a difficult period in which his regime has imprisoned bloggers, religious leaders and dissidents; curtailed labour laws; and again taken control of what one Vietnam expert called the "commanding heights" of the economy.
Obama referred gently to the issues, saying: "All of us have to respect issues like freedom of expression, freedom of religion, freedom of assembly. And we had a very candid conversation about both the progress that Vietnam is making and the challenges that remain."
Sang, sitting next to him in the Oval Office, mentioned the legacy of the Vietnam war and said "we still have differences" concerning his country's human rights record.
Before the two parted, Obama referred to a letter that Sang had shown him. In it, the president said, Ho Chi Minh expressed his hope to then president Harry Truman that Vietnam could co-operate with the US.
"President Sang indicated that even if it's 67 years later, it's good that we're still making progress," Obama said.
Agence France-Press, The New York Times