US President-elect Donald Trump accuses China of keeping its yuan currency artificially low and slapping unfair taxes on American exports.

However, analysts say Beijing is in fact struggling to prop up the yuan as capital flows out of China's flagging economy in search of better investments in the United States.

"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)?" Trump wrote Sunday in a Twitter outburst.

"I don't think so!"

- Close controls -

China keeps close controls on its currency, allowing it to fluctuate within a two percent margin from a point fixed by the central bank each day.

And its value has been steadily dropping for over a year.

The yuan, also known as the renminbi, is currently at an eight-year low of 6.89 to the dollar.

Yet the evidence suggests the currency is over, not under, valued.

On August 2015, the People's Bank of China announced a surprise five percent devaluation of the yuan. Since then, the currency has fallen another six percent.

The initial devaluation, Beijing said, was intended to make the yuan trade closer to its market value as the country lobbied to have the unit join the International Monetary Fund's elite basket of currencies -- not help exporters, as Trump claims.

But downward pressure proved to be stronger than expected, and since that time the PBoC has been scrambling to prop it up.

China is hemorrhaging foreign currency reserves. Data released on Wednesday show they plunged by $69 billion in November alone.

China has spent that money on buying back its own currency, analysts say, in an effort to keep the yuan's value artificially higher.

China prefers to see "the yuan strengthen, or at least be stable", to avoid volatility that compromises the currency's evolving internationalisation, Credit Agricole analyst Dariusz Kowalczyk said.

Meanwhile, the US dollar is soaring on expectations Trump will embark on an infrastructure spending spree that would fan inflation and force the Federal Reserve to hike interest rates.

That expectation is fuelling demand for US assets -- causing capital to flee China and other emerging markets.

Capital flight from China is estimated by Bloomberg at some $1 trillion in 2015 and continuing in 2016, despite recent efforts by Beijing to tighten restrictions on currency flows.

The stricter controls have concerned foreign companies, which are reportedly having trouble moving money out of China.

China's economy meanwhile is already growing at its slowest rate in a quarter of a century.

"I smell more capital controls (to come), which run against any hopes of reform or internationalisation of the yuan or a sharp drop in the yuan," said Michael Every, head of financial markets research at Rabobank NA in Hong Kong.

"And all these fun and games come before we have even seen President Trump."

- Protectionist threat -

Trump takes office in January, after loudly blaming China and free-trade deals for job losses and declining wages in America.

His pledge to ramp up import duties stems from his belief that China continues to "heavily tax our products going into their country".

Cars imported to China are subject to a tax of more than 25 percent, on top of which Beijing recently slapped an extra 10 percent levy targeting luxury vehicles.

The duties follow World Trade Organisation rules, China says.

Cars imported into the US are taxed around 2.5 percent -- but Trump has proposed raising this to 35 percent and possibly to 45 percent for products from China.

Looking to boost domestic consumption, in 2015 Beijing slashed tariffs on a range of imported products, from shoes to cosmetics and certain items of clothing.

But if Trump follows through on his plans to implement protectionist measures, none of that will save the Chinese currency, said Rabobank's Every: "The yuan has further to fall."