President-elect Donald Trump's vague and ambiguous foreign policy positions have cast a pall of uncertainty over whether American influence will decline in Asia, or if it will remain a force to be reckoned with, analysts say.

The real estate tycoon-turned-politician frequently savaged China on the campaign trail, even calling it America's "enemy" and pledging to stand up to a country he says views the US as a pushover.

But he has also indicated he is not interested in getting involved in far-off squabbles, saying America is sick of paying to defend allies like Japan and South Korea, even suggesting they should develop their own nuclear weapons.

"Trump could play the isolationist card and strike a deal with China to share regional influence," said Ashley Townshend of the United States Studies Centre at the University of Sydney.

"But he might equally decide to adopt a firm military stance on a country he thinks regards America as weak."

Trump has offered no clear prescriptions for the geopolitical issues that plague the relationship between Washington and Beijing, from Beijing's territorial claims in the South China Sea to North Korea's nuclear programme and the future of Taiwan.

"At this juncture, governments around the world cannot depend on any particular set of US policies, since Trump's sometimes flip foreign policy statements were often contradictory," said Graham Webster, a US-China expert at Yale Law School.

- Isolationist US? -

In recent months, despite President Barack Obama's foreign policy "pivot" to Asia the US has seen some of its regional allies begin to drift into Beijing's sphere of influence -- attracted by the economic appeal of the neighbourhood's biggest player.

Newly elected Philippines President Rodrigo Duterte cosied up to China during a trip to the country last month, and has threatened to sever military relations with Washington.

Malaysia, too, has seemingly begun to eye improving relations with the world's second-largest economy.

The prospect of an isolationist US under President Trump could quicken that trickle as the developing countries of Southeast Asia see Beijing -- with its fiscal largesse and huge consumer base -- as a better bet than a protectionist US.

Meanwhile, Trump's assertions that he will require Japan and South Korea to pay more for US defence assistance has led those countries, too, to worry about how the new presidency may reshape long-established relationships, said Rory Medcalf, head of the national security college at the Australian National University.

"Middle powers in Asia, like Australia, need to hedge against two problems now. One is Chinese power and the other is American unpredictability," he said.

But alongside the "America First" rhetoric of the campaign trail that seemed to signal a withdrawal from the world order, Trump has blustered that under him the US will once again be feared and respected by enemies and allies alike.

He has promised to increase US military strength -- boosting the navy to 350 ships -- and has spoken admiringly of the strongman politics of Russian President Vladimir Putin.

- 'Element of uncertainty' -

Just before the US election, Peter Navarro, who is said to be Trump's top China adviser, hinted at how the relationship with Beijing might change under the new president in an article on the website of Foreign Policy magazine.

The refocus on Asia under Obama has been a failure, he wrote.

The "weak pivot follow-through has invited Chinese aggression in the East and South China Seas", he said, adding that a Trump administration would address the problem, in part, by pursuing "a strategy of peace through strength".

In recent years Beijing has built a series of artificial islands capable of hosting military facilities in the South China Sea, an area stretching hundreds of kilometres (miles) from its shores, but which it claims as its own.

Littoral states have little capacity to resist themselves, but the US regards freedom of navigation in the strategically vital waters as a crucial issue and Washington has ordered periodic sail-pasts and fly-overs of disputed islands.

Beijing's official defence budget has seen annual double-digit expansions for most of the last two decades and it has the world's largest military at its command, with a second aircraft carrier under construction, although US forces remain more powerful.

Trump was an unconventional candidate, and his policy direction on the issues embroiling the US and China once he is sworn into office in January remains largely unknown.

"Trump has not expressed his position on the South China Sea problem," said Jia Qingguo, head of the Beijing University School of International Relations.

That and many other questions were so opaque, he said, that Beijing finds itself at a loss for how to proceed.

"Since Trump has been elected President, uncertainty in Sino-US relations has increased across the board," Jia said.

"I hope he will handle China-US relations in a pragmatic and rational way."