'Disagreements over territorial claims and the appropriate use of the maritime domain appear to be a growing challenge to regional stability and prosperity,' Gates said. 'Competing claims should be settled peacefully, without force or coercion, through collaborative diplomatic processes, and in keeping with customary international law.

'The United States has always exercised our rights and supported the rights of others to transit through, and operate in, international waters.

'This will not change, nor will our commitment to engage in activities and exercises together with our allies and partners.'

His words mirror statements by US Secretary of State Hillary Rodham Clinton during an earlier Asean security gathering in July that sparked an angry reaction from Foreign Minister Yang Jiechi and later protests from Beijing. PLA generals later told Vietnam that it would 'regret' its fast-emerging strategic partnership with its once bitter enemy, the US.

But just as other ministers held their ground then, so did others yesterday. Japan, Indonesia, Malaysia, Australia and New Zealand were among those that also raised concerns, along with host Vietnam - the country that most fears neighbouring China's assertiveness in the South China Sea.

As well as claiming much of the waters of the strategic and mineral rich sea, China also claims its Paracel and Spratly islands in their entirety. Vietnam also claims both island chains, while Malaysia, Brunei and the Philippines claim parts.

Tensions have soared over the last two years or so, with Beijing's envoys pressuring international oil giants to pull out of exploration deals in southern Vietnamese waters, Chinese coastguard ships arresting hundreds of Vietnamese fishermen and unprecedented PLA naval exercises.

The Malaysian and Indonesian navies have had near-misses with Chinese vessels, while the US has faced unprecedented challenges to surveillance operations by Chinese fishing fleets.

'The South China Sea was certainly a running theme,' one participant said. 'There wasn't the anger of the session a few months ago, but although we didn't have it on the formal agenda there was a sense that we needed to carefully create a pattern of expectations and of conduct. I think that was achieved.'

This time around, Beijing apparently did not dignify the concerns with a direct response. In his speech, Defence Minister General Liang Guanglie repeatedly stressed the peaceful and 'defensive' nature of China's policies, saying it was committed to promoting international and regional peace and security. But he also made clear that China was not keen on group negotiations to handle all sensitive or security issues - something the US and now Asean are increasingly pushing for. 'In our opinion,' Liang said, 'practical co-operation within multilateral frameworks does not mean settling all security issues, which is not consistent with the principles of gradualism and taking into account the comfort level of all parties.'

In the final press conference, Liang even left Vietnam's defence minister to answer a question directed at him over the South China Sea. Lieutenant General Phung Quang Thanh said that while the issue was not on the official agenda, the need for peaceful dialogue was discussed.

He said the meeting was keen to see progress on an Asean-led effort to hammer out a legally binding code of conduct with China to cover activities in the South China Sea, as well as settlements under the UN Law of the Sea.

Liang's spokesman, Rear Admiral Guan Youfei, was more blunt as he left the hall of Hanoi's National Convention Centre. He said the South China Sea issue had been 'not raised, but mentioned', so it was not up to China to respond. 'It is their problem,' he said of the countries that included it in their speeches. 'It is not our problem.'

The question for many insiders and observers is whether the recent flurry of military diplomacy will bring any practical changes.

The Association of Southeast Asian Nations has long favoured talk over hard action. But the new 'Asean plus eight' grouping - driven by Vietnam and with clear US backing - is trying to bring about practical co-operation on a range of issues, from maritime security to counterterrorism and disaster relief.

It seeks to have a greater role than the much derided Asean regional forum, a security gathering of foreign ministers rather than defence officials.

The new grouping links the defence chiefs of the 10 Asean countries with their counterparts from China, the US, Japan, South Korea, India, Russia, Australia and New Zealand. However, the ministers will meet only once every three years, with their senior staff meeting annually.