Ratings agency Standard & Poor’s cut Japan’s long-term debt rating on Thursday for the first time since 2002, and hours later Moody’s Investors Service warned the risk of the United States losing its top AAA rating, although small, was rising.

Bond markets reacted calmly, but the latest warnings about the colossal liabilities piled up by the two countries raised fears of rising borrowing costs that could hamper attempts to restore fiscal discipline and consolidate a fragile recovery.

“In advanced economies where fiscal sustainability has not been a market concern, credible plans going well beyond 2011 need to be put in place urgently to lock in benevolent market sentiment,” the IMF said in its “Fiscal Monitor” report.