TOKYO - Moody's Investors Service on Tuesday downgraded its outlook for Japan's credit rating because of concerns over its massive national debt.

The rating agency changed its outlook for Japan's Aa2 rating from stable to negative.

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In January, Standard & Poor's cut Japan's credit rating from AA to AA- for the first time in almost nine years due to concerns over ballooning debt.

Moody's said the downgrade was due to "increasing uncertainty" over Japan's ability to implement effective measures to rein in rising debt.

Japan's debt ratio is already among the highest in the developed world.

The finance ministry estimated in January that the country's public debt would swell to 997.7 trillion yen ($12 trillion) by March 2012, up from 943 trillion yen this year.