The ¥789.9 billion ($7.76 billion) surplus in the broadest measure of a nation's trade with the rest of the world was sharply lower than the ¥4.2 trillion surplus registered a year earlier, data from the Ministry of Finance showed Monday. Until recently, the country routinely produced a surplus in excess of ¥10 trillion a year.

When Mr. Abe came to power 17 months ago, he introduced an inflation target and called for aggressive monetary easing. That helped weaken the yen's export-crippling strength. But to his surprise, exports have yet to catch fire. So far the weaker yen has merely made imports more expensive at a time when the nation is more reliant on fossil fuel from other countries.

The trend of large surpluses abruptly reversed in 2011, when a nuclear accident forced the shutdown of nuclear plants across the nation, resulting in a surge in imports of fossil fuels.

Japanese manufacturers shifting their production offshore has also exacerbated the situation.

Any additional falls in the value of the yen could put further pressure on the current account, pushing it into deficit as returns on investments overseas fail to outweigh the trade deficit. That trade gap is also expected to widen further as the population ages, the workforce shrinks, and retirees multiply.

A current account deficit means a nation is spending more than it earns from trade and investment. A country that builds up liabilities with foreign creditors through chronic current account deficits could face funding problems if investors decide to withdraw their financing.

BNP Paribas predicts Japan will record an annual current account deficit next year, while Credit Suisse thinks it will happen after 2017.

"Some might hope that the income surplus will continue to widen at a pace sufficient to offset further deterioration in the trade balance," economists at Credit Suisse said. "But that could prove difficult in practice, given that the world economy is facing a decline in returns," they added, pointing to the global low rate environment and slower growth in emerging economies.

The recent deterioration in Japan's trade balance, as a result of slower exports and stronger imports, points to the possibility that this day of reckoning may come sooner rather than later.

"The recent surge in imports was a surprise," said Takashi Shiono, an economist with Credit Suisse. Some of it may be due to rush purchases ahead of the April 1 sales tax hike, but data for April suggest that a change is more permanent than temporary: Japanese are embracing imported items much more than before, he said.

For now, the Bank of Japan is effectively taking care of Japan's fiscal woes. By buying up Japanese government bonds at a faster pace than the government issues new debt, the central bank makes sure the supply-demand balance remains tight for JGBs. But with inflation rising on the back of a weakening yen, it's not clear how long the BOJ will be able to continue its ultra-easy monetary policy.

Having largely giving up on achieving a balanced budget, Japan's current goal is to stabilize the debt situation by 2020, mainly through sales tax increases, around the current debt-to-gross domestic product ratio of 240%. But according to a government panel report released on April 28, even that won't be enough to bring the situation completely under control beyond 2020, as baby boomers reach 75, an age associated with a huge increase in medical and elderly care expenses, much of which are funded by the central government.

Some economists argue that there is no need to treat debt owed to domestic and foreign investors differently. No matter who owns the debt, the government's job should be the same: keeping its debt under control and achieving healthy economic growth, said Takao Komine, professor of economics at Hosei University.

Mr. Komine points to the U.S., which has run twin deficits of fiscal and current account shortfalls for many years.

For the month of March, the current account balance came to a surplus of ¥116.4 billion, compared with a median forecast of a ¥294 billion surplus according to a survey conducted by The Wall Street Journal and the Nikkei.

The current account measures trade in goods, services, tourism and investment. It is calculated by determining the difference between Japan's income from foreign sources against payments on foreign obligations and excludes net capital investment.

 Mitsuru Obe at [email protected]

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