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U.S. Defense Secretary Leon Panetta used a visit Sunday to Vietnam to make clear Washington's intent to aid allies in the Asia-Pacific region develop and enforce maritime rights in the South China Sea, which Beijing largely claims.

On a historic stop in Cam Ranh Bay, the strategic deep water port that was a U.S. base during the Vietnam War, Panetta could gaze out from the flight deck of the USNS Richard E. Byrd toward the sea and reflect on the significance of the harbor, which represents both a painful past for the American military and a challenging but hopeful future.

"The new defense strategy that we have put in place for the United States represents a number of key elements that will be tested in the Asia-Pacific region," Panetta told reporters gathered under a blazing sun on the deck of the cargo vessel.

He said the U.S. would "work with our partners like Vietnam to be able to use harbors like this as we move our ships from our ports on the West Coast toward our stations here in the Pacific."

Panetta never mentioned China as he spoke to crew members on the Byrd and later to reporters. But with the South China Sea as a backdrop, he left no doubt that the U.S. will maintain a strong presence in the region and wants to help allies protect themselves and their maritime rights.

His visit, however, is likely to irritate Chinese leaders who are unhappy with any U.S. buildup in the region and view it as a possible threat.

Panetta, in remarks Saturday to a defense conference in Singapore, rejected such claims about the shift in U.S. military focus. But U.S. officials are wary of China's increased military buildup and expanding trade relations with other countries in the region.

"Access for United States naval ships into this facility is a key component of this relationship (with Vietnam) and we see a tremendous potential here for the future," he said.

This is Panetta's first visit to Vietnam, and his stop at the harbor made him the most senior U.S. official to go to Cam Ranh Bay since the Vietnam War ended.

Right now U.S. warships do not go into the harbor, but other Navy ships, such as the Byrd, do. The Byrd is a cargo ship operated by the Navy's Military Sealift Command; it has a largely civilian crew. It is used to move military supplies to U.S. forces around the world. Navy warships go to other Vietnam ports, such as Danang.

While Panetta suggested the U.S. may want to send more ships to Cam Ranh Bay in the future, he and other defense officials did not detail what requests he may make in meetings with Vietnamese leaders.

On Sunday, the port served more as a symbol of America's growing military relationship with Vietnam, underscoring Washington's desire to build partnerships in the region in part to counter China's escalating dominance.

For Panetta, who was in the military during the Vietnam era but did not serve in country, it was an emotional opportunity.

"For me personally this is a very moving moment," he said, noting that on Memorial Day he was at the Vietnam memorial in Washington commemorating the 50th anniversary of the war.

"Today I stand on a U.S. ship here in Cam Ranh Bayh Bay to recognize the 17th anniversary of the normalization of relations between the United States and Vietnam," he said.

The relationship between the two nations has come a long way, he said, "We have a complicated relationship but we're not bound by that history."

The new U.S. strategy for the Asia-Pacific includes a broad plan to help countries learn to better defend themselves, and for that to happen "it is very important that we be able to protect key maritime rights for all nations in the South China Sea and elsewhere," Panetta said from the deck of the ship.

China claims almost the entire South China Sea as its own, setting up conflicts with other nations in the region, including Vietnam, Taiwan, Malaysia, Singapore and others who also have territorial claims there.

Panetta flew to Vietnam from a major defense conference in Singapore, where he met with leaders from allies all across the region. There he issued a strong call for Asian nations to set up a code of conduct, including rules governing maritime rights and navigation in the South China Sea, and then develop a forum where disputes can be settled.

At the same time he detailed plans to boost U.S. military presence in the region, including a modest increase in ships and more troops that would mainly rotate in and out. Defense officials said that by 2020 the U.S. Navy would add about eight ships to the Asia-Pacific region, and overall would have about 60 percent of the fleet assigned there.

Tensions between the U.S. and China reverberate across the region, and are often focused on America's support of the island of Taiwan, which China considers its own. Another key area of dispute is the South China Sea, which China claims almost entirely as its own. But Taiwan, Vietnam, Malaysia, Singapore, Brunei and the Philippines also have territorial claims there.

In addition, more recently the U.S. has been vocal in blaming China for cyberattacks that emanate from the country and steal critical data from U.S. government agencies and private American companies.