The United States no longer has military primacy in the Pacific and could struggle to defend allies against China, a top Australian think tank has warned.

A hard-hitting report from the United States Studies Centre at the University of Sydney released on Monday said the US military is an "atrophying force" that is "dangerously overstretched" and "ill-prepared" for a confrontation with China.

If correct, the assessment has far-reaching implications for US allies like Australia, Taiwan and Japan who depend on American security guarantees.

Donald Trump's presidency has deepened concerns that Washington would not defend its allies in the face of aggression from China. But this latest report has suggested that the United States may struggle to help even if it wanted to.

Accusing Washington of "strategic insolvency", the authors said decades-long Middle East wars, partisanship and under-investment have left Pacific allies exposed.

"China, by contrast, is growing ever more capable of challenging the regional order by force as a result of its large-scale investment in advanced military systems," they warned.

Under President Xi Jinping, China's official defence budget has increased by around 75 percent to $178 billion -- although the true figure is believed to be much more.

Crucially, Beijing has invested in precision ballistic missiles and counter-intervention systems that would make it difficult for the US military to reach contested areas quickly.

According to the report, "almost all American, allied and partner bases, airstrips, ports and military installations in the Western Pacific" lack hardened infrastructure and are under major threat.

That advantage could be used to seize territory in Taiwan, Japanese-administered islands or the South China Sea before US forces could get there.

Experts believe that the deployment of US land-based missiles and a changed role for the United States Marine Corps will be vital to countering China, as well as collective regional defence -- with the likes of Australia and Japan doing more.

Story continues

In Australia, concerns have been growing about inadequate defences, prompting debate about whether the country should think about developing nuclear weapons.

Similar discussions are have periodically taken place in neighbouring Indonesia.

A separate report released on Sunday by the Australian Strategic Policy Institute recommended Australia boost and harden military capabilities in the thinly-populated north of the country.

"Because of the significantly reduced warning times for future conflict," wrote author John Coyne, it is likely the north of Australia will be used as a forward operating base or a "lily pad" to reach conflict zones.

The US military has already earmarked around US$210 million to boost a Marine Corps base near Darwin.

During a recent visit to Australia, Defense Secretary Mark Esper suggested the United States wants to deploy intermediate-range missiles in Asia. So far, the Australian government has stressed it has not received a formal request to host those weapons.