* BOJ says won't tolerate zero inflation, let alone deflation
* Surprises with tweak to its definition of price stability
* Shows BOJ deflation stance more in sync with government
* Governor Shirakawa: no commitment to ease policy further
* BOJ keeps rates on hold at 0.1 pct as expected (Adds details)
By Leika Kihara
TOKYO, Dec 18 (Reuters) - The Bank of Japan said it would not tolerate zero inflation or falling prices and reinforced its commitment to maintain very low interest rates, setting the scene for further easing of monetary policy to fight deflation.
The BOJ's most aggressive statement on deflation since the end of quantitative easing in 2006 sparked a surge in bond prices to factor in the risk of fresh policy action, such as expanding a new funding operation or increasing government bond purchases.
The commitment to fight falling prices also marks the central bank's second significant response in a matter of weeks to government pressure to help prevent deflation from tipping the economy back into a recession, after it offered short-term funding at an emergency meeting on Dec. 1.
While Governor Masaaki Shirakawa said the BOJ wasn't committing itself to ease policy further, failure to follow through with a clear plan to beat deflation may put its credibility on the line, especially since Japan has been in deflation for much of the past decade anyway.
"There had been the view that the BOJ was too optimistic about the outlook for the Japanese economy and the state of deflation, and markets had thought the BOJ might tolerate deflation. But it made clear that it doesn't, so that was good," said Susumu Kato, chief economist at Calyon Capital Markets Japan.
"But people may start having doubts about the credibility of the BOJ's stance unless it makes clear how it will cope with deflation."
IN SYNC WITH GOVERNMENT
As widely expected, the BOJ left its policy rate at 0.1 percent, a level that analysts forecast could stay unchanged until 2012 given deflationary pressures, in contrast to other major central banks which are expected to start raising rates later next year.
The lead March 10-year Japanese government bond futures finished near a 20-month high, while the five-year bond yield hit a four-year low of 0.445 percent.
The two-year JGB yield also fell 1 basis point to 0.160 percent, near a four-year low hit earlier in the week.
The BOJ has forecast at least three years of deflation, a consequence of Japan's longest recession since World War Two which ended earlier this year.
"This shows that the direction is at the very least toward a continuation of monetary easing, and also suggests there is the chance that monetary easing may be strengthened," said Naomi Hasegawa, senior fixed income strategist for Mitsubishi UFJ Securities.
The BOJ changed its definition of long-term price stability to show that it refuses to tolerate not only deflation but now flat consumer prices. Previously it had described price stability as consumer price growth of zero to 2 percent.
It was the first change to a definition the BOJ first applied in March 2006 to show its policy stance after ending quantitative easing following a bout of deflation for much of 1998 to 2006.
"It seems the BOJ wanted to get zero out of their price understanding. They're trying to show that they are going all out to stop deflation," said Adrian Foster, head of financial markets research at Rabobank International, Hong Kong.
"This is aimed at reinforcing that the BOJ and the finance ministry are in sync and that they don't want a stronger yen because of deflation."
Consumer prices have been falling for most of this year compared with year-earlier months. Data next week is expected to show that closely watched core consumer prices in November fell 1.7 percent from a year earlier.
MORE ACTION AHEAD?
The government, in office for just three months, is determined to keep the economy from slipping back into recession ahead of an election for parliament's upper house in mid-2010.
But with Japan's public debt nearing 200 percent of GDP, room for fiscal expansion is small. Indeed, the government is scrambling to cut spending plans for the next fiscal year under the threat of a Fitch ratings downgrade if it extends new bond issuance much beyond 44 trillion yen ($491 billion).
On the other hand, the BOJ has argued that there was little it could do to lift demand either since rates were near zero.
If Japan slips into another recession or concern over the country's huge public debt pushes up bond yields, the central bank may come under pressure for more aggressive policy action, such as increasing its purchases of government bonds.
"They are no doubt under pressure from the government to take some response, should the economy falter again," said David Cohen, an economist at Action Economics in Singapore.
The BOJ is virtually alone in expanding monetary easing. The Federal Reserve said on Wednesday it would roll back some of its emergency lending facilities in February and the European Central Bank has said it will start phasing out extra liquidity operations.
An increase in the BOJ's bond buying could push down the longer end of the yield curve. But the effect would be short-lived if it sticks to its current stance of focusing purchases on bonds that are close to maturity, analysts say.
The BOJ has been reluctant to increase long-term bond purchases beyond the current 21.6 trillion yen per year because the balance of its holdings is already near a self-imposed cap.
The central bank may instead boost its new fund-supplying tool announced on Dec. 1, either by expanding the lending period or increasing the amount to be offered from the current 10 trillion yen. ($1=89.66 Yen) (Additional reporting by Stanley White, Tetsushi Kajimoto; Editing by Neil Fullick)