Vietnam's President Truong Tan Sang accused China of making "groundless" claims of ownership of the vast South China Sea, saying the Philippines has the right to question the claims at the United Nations.
Speaking during a visit to Washington, Sang said Beijing's nine-dash line, which demarcates Chinese territories in the strategic and resource-rich South China Sea, lacks any legal foundation.
"The position of Vietnam is we always oppose the nine-dash line of China because it's a groundless claim—legally and practically," the Vietnamese leader said in answer to a question at a forum organized by the Washington-based Center for Strategic and International Studies (CSIS).
Sang then told experts at the CSIS that it would be helpful if they could determine whether China's claims are based on a strong legal foundation.
"We cannot find any legal foundation for the claim or scientific basis for such a claim," he said.
China's nine-dash line takes in about 90 percent of the 3.5 million square kilometer (1.35 million square mile) South China Sea on Chinese maps. It also cuts into the 200-mile (322-kilometer) exclusive economic zones of all the coastal states in the South China Sea.
A United Nations arbitration tribunal convened in The Hague last week to look into a complaint lodged by the Philippine government questioning the legality of China's claims.
The five-member tribunal under the U.N. Convention on the Law of the Sea has approved a set of rules to look into the legal challenge the Philippines launched against Beijing in January.
China and the Philippines, along with Brunei, Malaysia, Taiwan, and Vietnam, have overlapping claims across the South China Sea. Beijing claims nearly the entire sea.
Sang sidestepped a question on whether Vietnam will join the Philippines in international arbitration to resolve the territorial dispute, saying he did not want to give any comment on it.
He hastened to add however that as a U.N. member, the Philippines "has all legal rights to carry on with any proceedings they would like."
The Philippines and Vietnam have accused China of using assertive means to exert its claims and have rejected Beijing's map of the waters as a basis for joint development, a solution pushed by China.
Philippine ambassador to the United States Jose Cuisa, who was at the CSIS event, said deliberations at The Hague are expected to start "soon—in the next few weeks."
In its complaint, Manila argues that China's territorial claims, including occupation of several islets and reefs in the vast South China Sea, violate the U.N. Convention on the Law of the Sea and should be declared invalid.
The 1982 U.N. convention, ratified by more than 160 countries—including China and the Philippines—sets territorial limits for coastal states and aims to govern the use of offshore areas worldwide.
An inquiry will be triggered if the tribunal in The Hague declares it has jurisdiction over the case filed by the Philippines, reports have said.
China opposes attempts to involve third parties or world bodies in the South China Sea disputes, demanding one-on-one negotiations with other rival claimant governments.
Sang and U.S. President Barack Obama discussed the South China Sea issue in their talks at the White House Thursday and in a joint statement called for "the settlement of disputes by peaceful means" and renewed support for a code of conduct to manage potential mishaps.
Ahead of their talks, a group of 82 Vietnamese scholars, some long-time members of Vietnam's ruling Communist Party, sent an open letter to Sang, expressing their concern over Hanoi's relations with China.
They expressed concern over Chinese incursions in Vietnamese waters, despite Sang's visit to Beijing last month, during which the two countries agreed to set up a hotline to resolve fishing incidents in the disputed South China Sea.
"Even before the ink on the agreements mentioned in a joint communique could dry out, Chinese marine surveillance [teams] chased and attacked our fishermen’s boats near the Paracel Islands," the scholars said, asking Sang to use his Washington trip to "move away from China’s influence."