Chinese envoys are watching developments closely, fearing any US-Vietnam move that smacks of containment of a rising China. And future US access to Cam Ranh feeds directly into such fears.

For the third year in a row the resupply ship, USNS Richard E. Byrd, is undergoing 14 days of routine repairs around Cam Ranh. Figures from the US Military Sealift Command show it is the fifth US ship to be repaired in the area since the South China Morning Post first reported the Richard E Byrd's visit in 2010.

The two sides had agreed on four such visits, but the fact that a fifth ship is now being serviced shows some flexibility is already in place.

Professor Carl Thayer, a veteran Vietnam military scholar at the Australian Defence Force Academy, said he believed Panetta would be pushing hard for greater flexibility when he started talks in Hanoi on Monday.

'Both sides are approaching the wider relationship carefully, but in terms of Cam Ranh Bay, the US would love to be in a position where there is real flexibility, and they can visit for repairs as and when they feel they need to,' he said. 'This will be a key item of discussion.'

The Vietnamese newspaper Tuoi Tre this week quoted a local port official saying US ships would soon be visiting every two to three months. However, a Sealift Command spokesman said any future operations were matters of national security and could not be discussed.

The glittering prize of the cold war, Washington turned the deep-water port into a vast naval and air base to serve its defence of the then-South Vietnam. After Hanoi's forces won, Cam Ranh was handed to the former Soviet Union in the late 1970s.

Even before US-Vietnam ties were normalised in 1995, US admirals were talking up the strategic nature of Cam Ranh - the best natural harbour in East Asia - with one famously saying: 'We're always on the lookout for good ports.'

Yet, in other ways, the US repairs highlight lingering sensitivities in the relationship. The sealift ships are manned by civilians and considered non-combative - and therefore less provocative. They are also using civilian shipyards, rather than the military facilities now being rebuilt by Russian firms.

The Vietnamese government already announced that, in future, international navies would be welcome to use Cam Ranh - at market rates.

'It has long been anticipated the US will be first among equals when it comes to using Cam Ranh,' said one Asian military attache. 'It is now within their footprint - a fact not lost on Chinese strategic analysts.'

Thayer believes conservatives within Vietnam's Communist Party remain wary of US motives in driving political change, while in Washington, worries over human rights are stalling the evolution of a formal strategic partnership.

But Vietnam's fraternal relationship with neighbouring China is also a key brake on the wider friendship, according to both Vietnamese and foreign officials.

Beijing and Hanoi have been working hard to improve areas of the relationship - and Hanoi is wary of overplaying the US card.

US Assistant Secretary of State for East Asia, Kurt Campbell, acknowledged those fears during a visit to Hanoi earlier this year. He spoke of Washington supporting 'the strong relationship between Vietnam and China... and that we saw no circumstance where there should be a zero-sum set of circumstances involving the US, Vietnam and China.'