By Leika Kihara

TOKYO (Reuters) - The Bank of Japan could make negative interest rates the primary focus of its monetary policy on Wednesday, heightening market disquiet over what moving away from quantitative easing reveals about the waning firepower of global central banks.

With three years of massive money printing failing to push up inflation, the BOJ is expected to move away from shock therapy and toward a protracted battle against deflation, say sources familiar with its thinking.

Such policy modification will be at the center of a comprehensive assessment of its stimulus program the BOJ will conduct at the two-day rate review it begins on Tuesday.The BOJ's "quantitative and qualitative easing" (QQE) has been a signature policy of Governor Haruhiko Kuroda since 2013 that aimed to shock the economy out of stagnation and change households' deflationary mindsets.

While the central bank is unlikely to ditch QQE completely, altering its emphasis would herald an end to the "shock and awe" approach that made Kuroda's policies unique compared with the gradualist approach preferred by his predecessors.

A less aggressive approach would also come as a world of tame growth and low inflation force the U.S. Federal Reserve to go slow on raising interest rates and the European Central Bank to concede the limits of what monetary policy alone can achieve.

Policymakers got a taste last week of how markets might react when investors dumped longer-dated bonds on fears the BOJ would slow its purchasing pace.

Super-long Japanese government bond yields have rocketed since the BOJ announced at its July 28-29 meeting its plan to conduct the comprehensive review. The 20-year JGB yield, which hovered around 0.125 percent on July 29, hit a six-month high of 0.495 percent on Sept. 14.

The challenge for the BOJ will be how to back away from QQE without scaring investors into a stampede out of bonds.

"The BOJ insists that it still has many tools available. But the costs of using these tools are rising and the benefits are diminishing, especially for its huge asset purchases," said Izuru Kato, chief economist at Totan Research.

"Deepening negative rates has enormous costs too but practically, that's probably the only usable tool left."

The BOJ is also keen to dispel market concerns it is running out of ammunition to prevent unwelcome yen rises from hurting Japan's exports. The dollar may weaken against the yen if the Fed sends out a dovish message on its future rate hike path at its policy announcement due hours after the BOJ's, analysts say.

A growing belief the BOJ will stop short of the dramatic action needed to weaken its currency has weighed on the dollar, which hit a six-day low of 101.59 yen overnight on Monday.

ADMITTING DEFEAT?

Under QQE, the BOJ has been increasing base money - or the amount of money it prints - at an annual pace of 80 trillion yen ($783 billion). Analysts say the BOJ will struggle to buy enough bonds in coming years with its huge purchases draining liquidity.

Sources have told Reuters the BOJ will shift its prime policy target to the 0.1 percent negative rate it charges on a portion of excess reserves financial institutions park with the central bank that was introduced in February.

Wary of a flattening yield curve that risks impairing financial intermediation, it will also seek ways to steepen the curve such as making its bond buying more flexible.

But the BOJ must manage any such shift without giving markets the impression it could withdraw its massive stimulus.

Sources say the board may consider deepening negative rates to show its determination to maintain an ultra-easy policy bias, though analysts doubt whether that will do much to boost growth.

Kuroda must also muster consensus from a fragmented board, where even those who favor more easing are divided over the most effective method. Some want to shift to a rate target, but others want to keep focusing on expanding asset purchases.

"Kuroda created QQE, so it's his symbol. Making big changes to that would be admitting defeat," one of the sources said. "I wonder whether that's possible while Kuroda is governor."

($1 = 102.1300 yen)

(Reporting by Leika Kihara; Editing by Eric Meijer)