Moody's has put the United States under review, with the possibility of a downgrade in the country's AAA status.
Moody's is the first of the big three rating agencies to do so, which means it is close to cutting the rating.
In a statement, Moody's says it sees a rising possibility that the statutory debt limit will not be raised soon enough, leading to a default on US Treasury debt obligations.
Federal Reserve chairman Ben Bernanke said earlier that a default would send shockwaves through the entire financial system.
The BBC reports the United States hit its debt ceiling of $US14.3 trillion on 16 May. Republican party members refuse to lift the ceiling without deep government spending cuts.
