Haruhiko Kuroda, governor of the Bank of Japan, speaks during a news conference at the central bank's headquarters in Tokyo. Kuroda began his campaign to end 15 years of falling prices by doubling monthly bond purchases in a bid to reach 2 percent inflation in two years. (Haruyoshi Yamaguchi/Bloomberg)

Japan plans to flood its economy with freshly printed yen in coming months in an effort to shock one of the world’s major industrial powers out of a 20-year-old stupor.

The impact of an unexpectedly ambitious move by the Bank of Japan on Thursday could course quickly through the world economy, pulling down the value of Japan’s currency and with it the prices of the cars, electronics and other goods the country sells worldwide.

That could mean a larger U.S. trade deficit with the country and a more difficult market for American exports, which will become more expensive in Japan by comparison. The yen fell 3 percent against the dollar on Thursday, continuing a six-month-old trend in which it has dropped about 25 percent as the country prepared to shake up economic policy. On Friday, the yen fell again and Japan’s Nikkei 225 Stock Average soared, closing its morning session up 3.76 percent and climbing above 13,000 for the first time in almost five years.

Rivaling what the U.S. Federal Reserve did in the wake of the Lehman Brothers collapse, Japan’s move is the latest example of how the developed world is relying on its central bankers to keep the economy afloat while politicians grapple with underlying problems, including government debt and economic competitiveness.

Institutions such as the Fed, the European Central Bank and the Bank of Japan have waded deep into unfamiliar territory to do the job, buying government bonds and other assets while their own internal balance sheets balloon. They are pushing money into the economy with historic low interest rates to try to battle the fact that households are slow to spend and companies are hesitant to invest and hire.

Japan want to increase liquidity to spur its sluggish economy.

The risks are known but impossible to quantify: of inflation remaining tame until it roars out of control, or of asset bubbles creeping into unexpected parts of the economy as investors take advantage of cheap money worldwide to make ever-riskier bets.

Politicians, meanwhile, have made little progress on the underlying issues.

The U.S. budget debate is stalled, and Europe remains stuck in recession and confronting deep problems with government debt and the structure of its financial sector.

Japan’s ills also are complex — combining chronic issues such as demographic decline with acute problems such as the incomplete cleanup from the tsunami and earthquake in 2011 that displaced tens of thousands and damaged the Fukushima Daiichi nuclear complex.

The central bank action is meant to kick-start a broader push toward economic renewal promised by Prime Minister Shinzo Abe.

Calling the efforts of his predecessor “incremental,” new Bank of Japan governor Haruhiko Kuroda said he is prepared to pull out all the stops to boost growth and end Japan’s bout of falling wages and prices — a “deflation” that has mired households and companies in a self-fulfilling spiral of tight-fisted spending and downward expectations.

“This time, we took all necessary steps to achieve the target,” Kuroda said, according to wire service transcripts of his remarks in Tokyo. He referred to a planned program of $500 billion in asset purchases as “large beyond reason” but necessary to encourage households and investors to spend by increasing prices and pushing down bank lending rates. Rising prices tend to cause people to buy things before prices go up further.

In hopes of rekindling modest inflation in Japan within two years, the program would double the basic money supply in coming months — roughly what the Fed did from September 2008 to May 2009 during the worst of the U.S. crisis.

As in the United States and Europe, aggressive central bank action won’t fix Japan’s economy and can only be a stopgap for the Abe government to put the rest of its plan into motion, said Hung Tran, first deputy managing director at the Institute of International Finance.

“This will have an impact in the near term . . . but at the end of the day, the government has to come through,” Tran said, and address problems such as a drop in labor productivity, a decline in the number of workers, a collapse in domestic investment and savings, and other issues.

An aggressive move by the Bank of Japan has been expected since Abe’s election last year and his appointment of Kuroda to take over from a more conservative predecessor.

But the scale of action was beyond what many analysts foresaw. Although it could feed into perceptions of a developing “currency war” around the world, Japanese officials have been careful to lay the groundwork with their major trading partners in recent months — particularly the United States.

The country was cautioned this year not to include purchases of foreign government bonds or assets in its central bank plan — a step that would have been a direct intervention in currency and exchange rate markets. Talk of such a move stopped in Japan after major economic powers released a statement reaffirming the importance of free and floating exchange rates among major nations.

But the Abe government also has been clear that it plans major changes beyond what the Bank of Japan announced. If the short-term effort means pain for some of Japan’s trading partners, the longer-term vision — of a Japan that is growing, healthy and ready to open itself more fully to the rest of the world — should yield benefits.

In the short term, the country plans a major boost in government spending — another bridge before it discusses how to deal with its massive debt.

Abe has announced his intention to join the United States in the 11-nation Trans-Pacific Partnership trade negotiations — a diplomatic coup for an Obama administration hoping to expand that agreement.

U.S. Treasury officials said they had no comment on the Bank of Japan announcement.