Geng also said China would respect the liberty of ships and aircraft from 'relevant countries' traversing the South China Sea in accordance with international laws.

Until recently, Beijing would have limited itself to a routine expression of concern in the face of tensions over territorial claims in the waterway. This time it sent its three naval fleets - days after US Secretary of State Hillary Clinton declared at a regional summit in Hanoi that resolving the sea's territorial disputes was in the 'national interest' of the United States and a 'diplomatic priority'. The naval deployment appeared designed to showcase China's military advancement and its raw power, according to diplomats and analysts.

'We are seeing the region's big powers routinely flexing their muscles over the South China Sea at a very fluid moment,' said one veteran Asian military envoy. 'The neighbourhood is suddenly a very different place and we are entering a new era.'

China claims virtually the entire South China Sea and its strategic Paracel and Spratly archipelagoes. Vietnam also lays claim to the islands; Malaysia, the Philippines and Brunei lay claim to some of them.

Clinton's statements, backed by earlier warnings from US military officials, appear to support efforts by Vietnam to internationalise the territorial disputes in the face of growing efforts by China to suppress discussion of them in the region.

While analysts believe China may have planned this week's exercises well in advance, the fruits of two decades of double-digit military growth were given an extensive airing on domestic television. Its footage showed repeated missile firings from ships, submarines and fast-attack vessels in a highly co-ordinated combat scenario that also involved aircraft.

They come weeks after exercises in the East China Sea intended to pre-empt this week's large-scale joint drills by US and South Korean forces in the Sea of Japan, which Koreans call the East Sea.

Beijing is not alone in seeking to match words with action. The US, which has the most powerful military forces in Asia, is a master of the tactic. Weeks before Clinton arrived in Hanoi for the annual Asean Regional Forum on security, the Pentagon revealed three recently refitted large submarines armed with cruise missiles had, unusually, been deployed to the Asia-Pacific - one to the South China Sea.

Malaysia and Vietnam are increasing their naval presence in the area to match the growing US and Chinese military presence.

If Beijing aims to move beyond mere words and show its muscle, it must factor in the strategic shift in the region its displays of firepower has already brought about, with countries courting the US to re-engage in Southeast Asia - and finding a willing partner.

Clinton's statements at the Hanoi forum were part of a US-led ambush that saw 12 countries - including Japan, Indonesia, Vietnam and Australia - express concern about China's recent assertions of naval power and increased tensions in the South China Sea. Those concerns are expected to solidify ahead of the first gathering in October of defence ministers from Southeast Asia with eight regional powers that will now include the US and Russia.

Japan strengthened its stance this week, with its foreign minister, Katsuya Okada, warning that South China Sea issues could affect trade and stability. 'The instability deriving from differing views on territorial issues between China and Asean nations could undermine peace in Asia,' Okada said. 'We should get rid of such a destabilising factor as soon as possible.'

Veteran South China Sea watcher Professor Carl Thayer, of the Australian Defence Force Academy, believes several years of Chinese 'soft power' gains in the region are unravelling in the face of 'smart power' as the administration of US President Barack Obama crafts a fresh approach to East Asia. The events of the past few days had shown just how difficult that landscape was for China to navigate. 'Secretary Clinton has turned the multilateral table on China. The United States is back and engaged in Southeast Asia working with the support of regional states,' Thayer said.

'Continued Chinese bellicosity and diplomatic pique runs the risk of isolating China diplomatically and eroding the soft power gains of recent years. The timing is bad for China as the regional security architecture looks set to gain a new lease on life and expand into new areas of co-operation.'

Tensions are expected to rise further, with none of the key players likely to back away from their long-held positions. US military officials, for example, have already noted the need to expand patrols to reassert freedom of military passage in international waters close to China.

Shi Yinhong , a professor of international relations at Renmin University, said the series of exercises would increase tensions and harm ties with Washington.

'It is now difficult for the two countries to back down in the dispute as both sides put [the South China Sea] as their core national interest,' he said.

'The curious thing is that so far the Foreign Ministry has not commented on the exercises. It is impossible for the spokesman of the Ministry of Defence to come out and say Beijing is doing this to target the US.

'It is an obvious change. Just a month ago, when the PLA was conducting naval drills in the East China Sea, the Foreign Ministry on the very first day underlined that it was just a regular exercise and completely unrelated to the US-South Korea joint exercises.'

Andrei Chang, editor-in-chief of the Canada-based Kanwa Defence Review, said it was a big undertaking for the PLA to move its North Sea fleet warships from their base in Qingdao to the South China Sea.

'Obviously this has been under preparation for quite a long time. It could have been planned as early as June, when the US defence secretary, Robert Gates, expressed US concern over territorial disputes in the South China Sea at the Shangri-La Dialogue in Singapore,' Chang said. 'His speech upset many PLA generals.'

He said the PLA drills in the South China Sea, East Sea and even the Yellow Sea over the past month could be seen as 'military confrontations of national core interests between Beijing and Washington'.

Ni Lexiong , a Shanghai-based military expert, said the PLA naval fleets had tried to launch large-scale joint drills in the South China Sea several years ago.

'Beijing ... waited for a proper opportunity to hold military drills in these waters to show off its naval power and determination to defend the national interest,' he said. 'It worried that such a show of force would harm the neighbourhood ... But now the US has provided the PLA the best excuse to show its muscle.'