CAM RANH BAY, VIETNAM - Forty-five years ago, American cargo ships filled this vast harbor, unloading supplies day after day for U.S. troops fighting the Viet Cong.

Today, the bay's azure waters are largely empty, except for local fishing boats. The once-bustling U.S. air base here, formerly home to fighter squadrons and a combat hospital, is abandoned, a reminder of the U.S. military's exit from most of Southeast Asia after the Vietnam War.

But the Pentagon is plotting a return.

Sunday, Defense Secretary Leon E. Panetta flew to Cam Ranh Bay, the first Pentagon chief to come to this deep-water port 200 miles northeast of Ho Chi Minh City since the war. He recalled "the great deal of blood that was spilled in this war on all sides -- by Americans and by Vietnamese."

He also made clear that the United States hopes difficult history will not stand in the way of a U.S. return to the sheltered anchorage off the strategically important South China Sea.

"Access for United States naval ships into this facility is a key component" of the U.S. relationship with Vietnam, "and we see the tremendous potential here," Panetta said as he stood on the stern of a gray-hulled U.S. Navy supply ship undergoing maintenance.

The vessel is one of only a few U.S. ships the Vietnamese have allowed back to Cam Ranh Bay since diplomatic ties were re-established in 1995. But it is unarmed and sails with a largely civilian crew, a requirement imposed by the Vietnamese government that has prohibited military ships from docking since 2002.

U.S. warships have called regularly at other Vietnamese ports since the guided missile frigate Vandergrift made a port call in Hanoi in 2003.

The Obama administration is reasserting the U.S. role as a Pacific power. Seeking to counter China's growing military might, Pentagon planners are pursuing closer ties to countries on China's periphery and access to ports and other facilities to increase the U.S. presence.

Cam Ranh Bay is ideally located off the South China Sea. But a Vietnamese military officer accompanying Panetta said opening it to U.S. warships was not possible because the port was a "restricted military area."

He said he was a teenager in Hanoi during the war with the United States and did not fight. Most Vietnamese no longer view the United States as an enemy, but memories of the war remain strong for "the older generation," he said.

Panetta said being allowed to use such ports as Cam Ranh Bay is important to the new U.S. strategy, which relies on rotating ships, troops and other military equipment into the region from the United States, rather than establishing permanent bases, as it did during the Cold War.

Vietnam isn't eager to grant permission to re-establish a U.S. military presence. The relationship will develop "at its own pace," said a Defense Department official.

As a fallback, the Pentagon is considering asking the Philippines to reopen Subic Bay naval base and Clark airfield, two Cold War-era facilities also near the South China Sea. Vietnam views China's burgeoning military power as a threat and Hanoi has accused Beijing of sabotaging oil explorations twice last year by cutting undersea cables, a charge China denies.

Vietnam is nervous about antagonizing China further by forging too close a military relationship with Washington, U.S. officials acknowledged.