Following plans of massive joint military drills with Seoul in China's near waters, the United States is prepared for a new demonstration of military ties with Vietnam on Thursday, amid warnings from Beijing for the US to keep away from the region.

Though some experts said the military exchange between Washington and its former foe has limited impact on China, others are worried that Washington is shifting its China policy based on a wrong judgment of Beijing's strategic intentions.

The US destroyer USS John S. McCain docked on Tuesday in central Vietnam for a four-day exchange program with the Vietnamese navy.

"On the 12th we will have the first ever training exchanges with the Vietnamese navy on damage control, emergency repair and fire fighting," Mike Morley, the ship's public affairs officer, was quoted by AFP as saying.

On Sunday, the aircraft carrier USS George Washington, which is scheduled to take part in military exercises with Seoul in the Yellow Sea soon and hence put the Chinese capital in its striking range, already hosted a delegation of Vietnamese military in the South China Sea.

China insists on complete sovereignty over islands in the South China Sea, while the Philippines, Vietnam, Malaysia and Brunei have competing claims. To solve the dispute Beijing has suggested joint development of the resource-rich waters.

Hanoi, however, has been particularly vocal about the issue and at the same time grown increasingly closer to the US, from trade and commerce to negotiating a controversial deal to share civilian nuclear fuel and technology that could allow Vietnam to enrich uranium on its own territory.

It also joined the US in harsh attacks on China on the issue at the ASEAN annual forum last month, where US Secretary of State Hillary Clinton suggested an international mechanism to solve the South China Sea issue, hinting at US involvement.

But Philippine Foreign Secretary Alberto Romulo on Monday told Washington to keep out of the regional issue.

"It's ASEAN and China. Can I make myself clear? It's ASEAN and China. Is that clear enough?" he told reporters.

Southeast Asian countries are in a state of conflict on the issue, said Niu Xinchun, an expert on US studies with the China Institute of Contemporary International Relations.

"On security, they rely on the US but their economies are dependent on China. They do not want to see the Sino-US conflict continue and make them choose one," he said.

Yuan Zheng, a researcher for American studies with Chinese Academy of Social Science, said China does not need to worry about the "limited impact" of the US-Vietnam military exchange.

But Seoul's Chosun Ilbo commented on Tuesday in a report on the US-Vietnam cooperation that "for national interests, even foes with history of bloody wars in the Cold War might hold hands again without hesitation."

Pang Zhongying, a scholar on world politics with Beijing-based Renmin University of China, said there are several factors that will determine how far a US-Vietnam relationship can go.

"The two countries have disputes on political systems, human rights and of course the painful war memories," Pang said. "Vietnam, like China, faces the same America."

Based on the heavy US presence surrounding China - from the Republic of Korea and Japan to Vietnam and other ASEAN countries - Pang warned it seems Washington is changing its judgment on China.

"The declaration of a peaceful rise China made in 2005 was widely seen as a commitment to the US that Beijing would not challenge it, but it seems now the White House is not quite sure about that."

As a result, US President Obama is likely to make a shift from the Bush administration's China policies.

"I believe the strained Sino-US relation will continue in the whole second half of the year," Pang said.