The US has had the outlook on its sovereign debt cut to "negative" by the ratings agency Standard & Poor's (S&P).
The move, which casts doubt on US ability to tackle its huge debt and fiscal deficits, depressed shares around the world.
Other countries with the same coveted AAA rating had taken firm action to deal with their deficits, S&P officials said, but the US remained locked in a political battle that only fuelled further deterioration.
"Because ... the path to addressing these [problems] is not clear to us, we have revised our outlook on the long-term rating to negative from stable," S&P said.
While France, Germany and Britain all moved last year on their fiscal problems, "the US has yet to agree on a plan", S&P's Nikola Swann said.
The negative outlook meant there was a one-in-three chance the world's largest economy could lose its AAA rating within two years, for the first time ever, he said.
Leading indexes in the US and Europe were all down by about two per cent following the announcement.