* China Dec FX reserves fall to near $3 trln, seen as key level

* Reserves sink nearly $320 bln in 2016 as yuan weakens

* China stepping up measures to support currency, curb outflows

* Trump's China threats adding to pressure on yuan (Adds comments from China's forex regulator, Capital Economics estimates on outflows, fund manager) By Cheng Fang and Sue-Lin Wong BEIJING, Jan 7 (Reuters) - China's foreign exchange reserves fell to near six-year lows in December, but held just above the critical $3 trillion level, as authorities stepped in to support the weakening yuan ahead of U.S. President-elect Donald Trump's inauguration.

China's reserves shrank by $41 billion in December, slightly less than feared but the sixth straight month of declines, data showed on Saturday, after a week in which Beijing moved aggressively to punish those betting against the currency and make it harder for money to get out of the country.

Analysts had forecast a drop of $51 billion.

For the year as a whole, China's reserves fell nearly $320 billion to $3.011 trillion, on top of a record drop of $513 billion in 2015.

While the $3 trillion mark is not seen as a firm "line in the sand" for Beijing, concerns are swirling in global financial markets over the speed with which the country is depleting its ammunition to defend the currency and staunch capital outflows.

Some analysts estimate it needs to retain a minimum of $2.6 trillion to $2.8 trillion under the International Monetary Fund's (IMF's) adequacy measures.

If pressure on the yuan persists, analysts suspect China will continue to tighten the screws on outflows via administrative and regulatory means, while pouncing sporadically on short sellers in forex markets to discourage them from building up excessive bets against the currency.

But if it continues to burn through reserves at a rapid rate, some strategists believe China's leaders may have little choice but to sanction another big "one-off" devaluation like that in 2015, which would likely roil global financial markets and stoke tensions with the new Trump administration.

The yuan depreciated 6.6 percent against the surging dollar in 2016, its biggest one-year loss since 1994, and is expected to weaken further this year if the dollar's rally has legs.

Adding to the pressure, Trump has vowed to label China a currency manipulator on his first day in office, and has threatened to slap huge tariffs on imports of Chinese goods.

That has left Chinese eager to get money out of the country, creating what some researchers describe as a potentially destructive negative feedback loop, where fears of further yuan falls spur outflows that pile fresh pressure on the currency.