The Bank of Japan followed its US and European counterparts Wednesday, announcing extra bond buying to take its total monetary easing effort past $1 trillion as it seeks to revitalise the economy.

In a move that took some players by surprise the BoJ said it would extend its asset-purchasing scheme by 10 trillion yen ($128 billion), injecting more cash into a system where interest rates are already at or near zero.

The announcement, which came after a two-day meeting, sent the yen sliding against the dollar and euro, while the Nikkei surged to a four-month high.

The move will see the asset-buying programme -- the main tool for monetary easing that provides liquidity to markets when the bank purchases government and corporate bonds, and commercial paper -- rise to 80 trillion yen.

Finance Minister Jun Azumi welcomed the BoJ decision, saying that the bank "took more action than we anticipated" and it would help to support the economy.

The announcement comes as Japan's export-dependent economy struggles to right itself following a series of problems, including the March 2011 earthquake and tsunami disaster, the European debt crisis, slowing global demand and the strong yen.

A bleaker assessment of the state of things was a factor behind the move, with the bank downgrading its assessment of the economy for September, saying "the pick-up in economic activity has come to a pause".

Pressure on the central bank to take additional easing steps had been increasing after the European and US central banks moved to support their economies.

At the start of the month the European Central Bank announced a plan to buy sovereign debt to lower borrowing costs for weaker nations, brushing aside German opposition to unleash a so-called "big bazooka" against the debt crisis.

A week later the Federal Reserve unveiled a new bond-buying programme that it said it would not back away from until the economy was on the right track and unemployment was falling.

On foreign exchange markets the dollar rose against the yen, changing hands at 79.10 yen in Tokyo afternoon trade, up from 78.65 before the BoJ announcement. It lost ground later, changing hands at 78.91 in London.

The euro also gained, hitting 103.50 yen, up from 102.54 yen in morning trade, but sliding back to 102.62 during the European day.

Tokyo shares jumped, with the benchmark Nikkei 225 index climbing 1.62 percent to its highest level since May. They fell back slightly to close 1.19 percent up.

In its statement Wednesday the BoJ warned "the pick-up in economic activity has come to a pause" in Japan due to the slowing overseas economies.

"There remains a high degree of uncertainty about the global economy, including the prospects for the European debt problem, the momentum toward recovery for the US economy," it said.

"Attention should be given to developments" in global financial markets amid long-lagging European debt problems, it said.

Daiju Aoki, an economist at UBS, said the timing of the bank's move had come as a surprise.

He said many market players had expected an easing in October when the BoJ is expected to lower its growth estimate in a twice-a-year economic outlook report.

"The earlier-than-expected timing boosted markets," he said.

But Aoki noted the market was expecting some kind of further easing such as an expansion of purchases of long-term government bonds.

Wednesday's announcement will see 5 trillion yen spent on long-term bonds by the end of December 2013 and another 5 trillion yen on short-term bills by the end of June of next year.

As they will be purchased on the secondary market, the long-term bonds could have less than their full maturity remaining.

"It would be different if the timescale was for the end of this year," said Aoki.

"The decision today affected sentiment but I'm doubtful if it will have a rapid impact on the real economy," he said.