President-elect Donald Trump has fired off another Twitter broadside, attacking China over alleged currency manipulation and foreign policy, as the world waits to see who he will pick for the vital role of secretary of state.

Trump will have "a very full slate of meetings" on Monday as he looks to finalise key cabinet positions, senior aide Kellyanne Conway said Sunday.

America's friends and foes alike are keenly awaiting Trump's choice for the top diplomat role, hopeful that it will offer clues to the direction US policy will take after he is sworn in on January 20.

Based on Trump's Twitter activity on Sunday, relations with America's top trading partner may be headed for a downturn, with the businessman-turned-politician accusing Beijing of expansionism and of fiddling the exchange rate.

"Did China ask us if it was OK to devalue their currency (making it hard for our companies to compete), heavily tax our products going into their country (the US doesn't tax them) or to build a massive military complex in the middle of the South China Sea?" he demanded, adding: "I don't think so!"

The taunt came two days after Trump provoked a rebuke from China by accepting a call from the president of Taiwan -- the first such call in around four decades.

China regards self-ruling Taiwan as part of its own territory awaiting reunification, and any US move implying support for independence is gravely offensive to Beijing.

Washington does not formally recognise Taipei, and officially cleaves to a "One China" policy that says Beijing is the legitimate government.

In practice, the island enjoys many of the trappings of a full diplomatic relationship with the US.

Trump's incoming vice president, Mike Pence, played down the call's significance, describing it as a courtesy, and said any new policy on China would be decided after his inauguration.

However, The Washington Post reported Sunday that the call had been in the works for weeks, intended to signal a major shift in US policies toward Taiwan and China. The article cited people involved in planning the call.

- Trade war threat -

China was a frequent target for Trump during his presidential campaign and every sign points to his taking an aggressive line.

US politicians often accuse China of artificially depressing its currency, the renminbi, in order to boost its exports -- its value has fallen by around 15 percent in the past two-and-half years.

Trump has vowed to declare China a "currency manipulator" on the first day of his presidency, which would oblige the US Treasury to open negotiations with Beijing on the renminbi.

With China holding about a trillion dollars in US government debt, Washington would have little leverage in such talks, but the declaration would harm ties and boost the prospect of a trade war.

It is not yet clear whether Trump intends to recruit someone with greater diplomatic experience for the State Department role, but he has run the rule over several high-profile candidates.

Four names have been in circulation for weeks: former New York mayor Rudy Giuliani, former US ambassador to the UN John Bolton, former Republican presidential candidate Mitt Romney and retired army general and ex-CIA chief David Petraeus.

But now more suggestions have begun to emerge.

"It is true that he's broadened the search," Conway told reporters at Trump Tower in New York.

Trump's former campaign manager said the eventual nominee must be ready to "implement and adhere to the president-elect's America First foreign policy, if you will, his view of the world."

Former Utah governor and ambassador to Beijing Jon Huntsman is also in the mix, according to CNN, while other reports said Exxon CEO Rex Tillerson and Republican Senator Bob Corker are under consideration.

"That list is expanding because... there is not a finite list of finalists," Conway said.

- Petraeus is keen -

Petraeus, who resigned in disgrace as head of the CIA in 2012 after he was caught sharing secrets with his mistress, said he has paid for his mistakes and is ready to work for Trump.

The 64-year-old scholar-warrior, who led the widely-praised "surge" in Iraq from 2008 to 2010, has a depth of experience in world affairs unmatched by the other known candidates.

He pleaded guilty in 2015 to a misdemeanor charge of mishandling classified materials after sharing Afghan war logs with his lover. He was put on two years' probation and fined $100,000.

Pence praised Petraeus as "an American hero" on NBC's "Meet the Press," adding that he "made mistakes and he paid for his mistakes."

Trump, he added, "will factor the totality of general Petraeus's career in making this decision."

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