Japan has been warned by a global ratings agency and the International Monetary Fund about its heavy debt load.

The IMF says Japan's debt is on an “unsustainable” path that could cause a sharp rise in the cost of borrowing. An IMF report says Japan's troubles carry risks to the domestic and global economy.

Fitch rating agency says Japan has been able to sustain its large debts so far because its citizens are willing to buy large quantities of government bonds at low interest rates.

But the agency warns that an aging population and declining domestic savings could reduce that source of funds. That could worry other potential lenders, and prompt them to raise the interest rate on Japanese debt.