A United States nuclear aircraft carrier cruised in waters off Vietnam on Sunday as Hanoi once again challenged China's sovereignty over the Xisha Islands.

The Vietnamese government's latest high-profile attempt comes amid improving relationships between it and the US, experts noted.

According to the Associated Press, the USS George Washington sat off Vietnam's central coast to commemorate last month's 15th anniversary of normalized diplomatic relations between the former foes.

Echoing the stance of the US on the issue of the South China Sea, Vietnamese foreign ministry spokeswoman Nguyen Phuong Nga said on Thursday that Chinese vessels have been conducting seismic exploration since late May near the Xisha Islands, accusing China of "violating Vietnam's sovereignty (and) its sovereign rights in the South China Sea."

Chinese Foreign Ministry spokeswoman Jiang Yu said in a press release posted on the ministry's website on Friday that China firmly opposes any remarks and actions that violate its sovereignty over the Xisha Islands and adjacent waters in the South China Sea.

"China has indisputable sovereignty over the Xisha Islands and adjacent waters," Jiang stressed. Su Hao, director of the Strategy and Conflict Management Research Center at China Foreign Affairs University, told China Daily that international law and history give China sovereignty over the area.

"China was the first to have discovered and named the Xisha Islands in the South China Sea and also the first to have officially placed the South China Sea islands under the jurisdiction of its government," he said.

"Actually, Chinese vessels have been doing research in the Xisha area for a long time. It is not the first time," said Su.

Xu Liping from the Chinese Academy of Social Sciences said there was no question of China's sovereignty over the Xisha Islands and adjacent waters when Vietnam and China established diplomatic relations. The Vietnamese are now challenging the status quo with no historical and legal backing at all and it's remarks coincided with the position of the US, Su added.

Last month at the ASEAN Regional Forum in the Vietnamese capital of Hanoi, US Secretary of State Hillary Rodham Clinton said resolving disputes over the South China Sea was "pivotal" to regional stability and suggested an international mechanism to solve the issue. "Vietnam has brought the South China Sea issue up intentionally, aiming to internationalize the issue, so that it can act as a stronger counterweight to China, backed by the US," said Su.

"The US needs Vietnam as one of its tools to counter-balance China's development," said Xu, "But in my opinion, the two countries will not get too close, they are just taking what they need from each other at the moment."