The yen rallied against the dollar after the Bank of Japan's announcement

Japan’s central bank tweaked its longstanding monetary easing programme on Tuesday, in a surprise move that saw the yen strengthen quickly against the dollar while Tokyo stock markets fell.

After a two-day policy meeting, the bank said it would widen the band in which it would allow rates for 10-year Japan government bonds to move, saying it would “improve market functioning”.

“The Bank will expand the range of 10-year JGB yield fluctuations from the target level: from between around plus and minus 0.25 percentage points to between around plus and minus 0.5 percentage points,” it said in a statement.

The move saw the yen strengthen rapidly against the dollar, with the greenback falling from a daily high of 137 yen to 133 within minutes of the decision.

The announcement came during the morning break in Tokyo trade, but the key Nikkei 225 index plunged as it reopened, falling more than two percent.

The Bank of Japan left the rest of its longstanding loose monetary policy intact, including its years-old inflation target of two percent.

Governor Haruhiko Kuroda, whose term ends next spring, has for years struggled to steer the world’s third largest economy towards sustained two percent inflation, seen as necessary for growth.

Prices in Japan have risen sharply this year, with the consumer price index in October at 3.6 percent, the highest in four decades.

But Kuroda and the central bank consider the increases temporary, citing a lack of strong demand and wage rises.

Still, the BoJ has come under pressure to move away from its ultra-loose policy as central banks in other major economies hike interest rates to tackle inflation.

The resulting differential has seen the yen nosedive about 20 percent against the dollar this year.