It's artificial islands, like this one planned by China, that are causing alarm around the South China Sea

A powerful American naval officer has fired a warning at China for rapidly building "a Great Wall" of artificial islands across hotly-contested waters.

Admiral Harry Harris, soon to take charge of Pacific Command, told a dinner at the Australian War Memorial on Tuesday night that the string of new islands posed a serious threat to stability in the South China Sea.

He said the artificial expanse was "roughly the size of Canberra's Black Mountain Nature Reserve" and that they stretched across some of the world's busiest sea lanes.

Those sea lanes carry around 60 per cent of Australian trade, posing a major strategic conundrum for the Abbott government.

The comments by Admiral Harris are by far the most strident and colourful on the subject by a senior American leader.

They show the US pushing back against China's assertive president, Xi Jinping, who had been seen to be "winning" a contest for maritime dominance at the expense of its neighbours. 

His  speech also poses a major test for Australia as it endeavours to engage in good relations with its major trading partner, China, while "hedging" against security risks by drawing closer to the US and other partners in the region. 

One of the new islands in question is a runway and port-shaped structure extended more than 3km, over previously submerged coral reef, which analysts say could mark a tipping point in China's ability to project air power thousands of kilometres from its coastal waters.

"China is creating a great wall of sand with dredges and bulldozers over the course of months," said Admiral Harris, who is currently commander of the US Pacific Fleet.

"When one looks at China's pattern of provocative actions towards smaller claimant states, the lack of clarity on its sweeping nine-dash line claim that is inconsistent with international law, and the deep asymmetry between China's capabilities and those of its smaller neighbours – well, it's no surprise that the scope and pace of building man-made islands raises serious questions about Chinese intentions," he said. 

China has repeatedly rejected regional concerns, saying the constructions are "necessarily" and are taking place on Chinese territory. 

Michael Wesley, director of the Asia Pacific school at the Australian National University, said the speech shows how the US has been stung by claims that it "capitulated" to early rounds of Chinese coercion at the expense of an ally, the Philippines.

"This marks a real ramping up of US determination and resolve in the region, reflecting a realisation that China is determined to play hard ball in the South China Sea," said Professor Wesley.

He said Australia could not  avoid being affected "given that 60 per cent of its trade goes through the South China Sea". 

Professor Wesley said the strident American rhetoric marked a "dangerous escalation". 

However Prime Minister Tony Abbott, who has treated the South China Sea as a top-line strategic challenge, is likely to welcome the US intervention as a means of showing support for regional allies and partners and discouraging future Chinese misbehaviour.

Earlier this month, Mr Abbott signed a landmark security co-operation agreement with Vietnam, Australia's former wartime enemy, to "support freedom of navigation by air and by sea in the South China Sea".

"We both deplore any unilateral change to the status quo," said Mr Abbott, without mentioning China by name. 

It followed similar arrangements India and Japan and a series of lower-level co-operation moves with south-east Asian nations. 

Admiral Harris warned in his speech to the Australian Strategic Policy Institute about the increasing potential for "miscalculation" between regional players.

 He urged China and other nations to conform to a China-ASEAN code of conduct, which commits nations to exercise self-restraint.

 "How China proceeds will be a key indicator of whether the region is heading towards confrontation or co-operation," he said. Sydney Morning Herald