* Exports up 12.1 pct in December yr/yr, more than forecast
* China overtakes U.S. as Japan's top export market in 2009
* China tightening, yen's rise pose risks (Adds more comments, background)
By Hideyuki Sano
TOKYO, Jan 27 (Reuters) - Japan's exports in December rose from a year earlier for the first time since before the financial crisis hit, but a rising yen and a growing reliance on Chinese demand may bode ill as Beijing looks to tighten monetary policy.
The bigger than expected 12.1 percent rise in exports came largely thanks to a jump in demand from Asia, which took in more than half of Japan's total exports in the month.
"As Chinese authorities are trying to rein in overheating in the economy, export growth will probably slow down a bit," said Hiroshi Watanabe, senior economist at Daiwa Institute of Research.
Still, Japan's export strength -- the benefits of which have had little impact on households or the domestic sector -- looks likely to keep the economy on the path to recovery for now.
"Japan's economic growth is likely to accelerate towards the end of this year, reducing the possibility of the BOJ taking further easing measures," said Kyohei Morita, chief Japan economist at Barclays Capital.
"But given that the yen is likely to appreciate again on such factors as China's possible tightening, the BOJ may be prompted to take steps to respond to market volatility."
The yen hit a six-week high around 89.35 yen to the dollar on Wednesday, getting closer to the 14-year high of less than 85 yen per dollar hit in November.
The rise in exports in December beat a median market forecast of a 7.6 percent gain and marked the first annual increase in 15 months, Ministry of Finance trade data showed on Wednesday.
Economists said the headline figure suggested exports grew solidly from November and shipments of machines, which had been slow to recover, were picking up.
China replaced the U.S. as Japan's biggest export destination for the first time in 2009, Finance Ministry data showed.
Exports to China rose 42.8 percent on the back of strong growth in the country, which grew 10.7 percent from a year earlier in the three months to December.
Financial markets have been spooked by worries that growth in Chinese demand could slow as Chinese authorities start pulling in the reins on loans and liquidity to stave off inflationary pressures.
Exports to the European Union also posted their first annal rise in 17 months.
In contrast, shipments to the United States fell 7.6 percent, a sign that improvements in the global economy remained uneven.
"Most countries around the world are seeing their economies recover but improvements in advanced economies remain fragile," said Takeshi Minami, chief economist at Norinchukin Research Institute.
"We can't rely too much on strong growth in those countries, so Japanese exports will continue to focus on shipments to Asia."
The Nikkei share average briefly hit a five-week low as investors around the world dump shares on worries that the global economy may not be ready for less stimulus just yet.
The yen's surge in November, coupled with government pressure for action to beat deflation, prompted the BOJ to put in place a new funding operation in December. (Editing by Hugh Lawson)