Vietnamese authorities have officially opposed China's establishment of Sansha City which overlaps Vietnam's Truong Sa (Spratly) and Hoang Sa (Paracel) archipelagos.
The Vietnam News Agency quoted leaders of Da Nang City and Khanh Hoa Province as saying that China has "seriously violated Vietnam's sovereignty" and that the city has "no legal legitimacy."
They said the two archipelagoes are parts of Vietnam and under the administrative management of Da Nang and Khanh Hoa.
The governments and people of Da Nang and Khanh Hoa, therefore, "are frustrated by China's decision to establish the Sansha City" that includes Hoang Sa and Truong Sa Districts.
Nguyen Chien Thang, Chairman of Khanh Hoa Province People's Committee, said China's decision "has seriously violated Vietnam's sovereignty and has no legal value.
"We oppose this and request that China cancel this wrongful and unreasonable decision, and take no further steps that could affect bilateral relations and the friendship between the two countries," he said.
The Vietnamese National Assembly on June 21 passed the Law on the Sea of Vietnam, which was later criticized by the Chinese government.
In response to the criticism, Luong Thanh Nghi, spokesman of the Vietnamese Ministry of Foreign Affairs said on the same day that the law aimed at completing Vietnam's legal framework over its sea sovereignty and enhancing its cooperation with other countries for the sake of peace and stability in the region and the world.
"It is regrettable that China reacted with unreasonable criticism against Vietnam's legitimate conduct. It is even more serious that China has approved the establishment of the so-called "˜Sansha City' that put Vietnam's Hoang Sa and Truong Sa archipelagos under its jurisdiction.
"Vietnam resolutely rejects China's unreasonable criticism as well as strongly opposes the establishment of the so-called "˜Sansha City'," Nghi said.
"It should be reiterated that Vietnam has sufficient legal basis and historical evidence to assert its sovereignty over Hoang Sa and Truong Sa archipelagos," Nghi added.
He said the inscription of Hoang Sa and Truong Sa archipelagos into the Law on the Sea of Vietnam is the continuation of a number of provisions with existing Vietnamese law.
"This is not a new matter and will not affect the search for fundamental and long-term solutions to the disputes in the East Sea. Vietnam consistently advocates settlement of differences and disputes in the East Sea by peaceful means and on the basis of international laws, especially the 1982 United Nations Convention on the Law of the Sea and the Declaration on the Conduct of Parties in the South China Sea (DOC).
"Vietnam always attaches importance to its relations with China and stands ready together with China to enhance the bilateral comprehensive strategic cooperative partnership for peace, stability and cooperation in the region and the world," Nghi said in a statement.