The US Treasury has hit back against a Standard and Poor's downgrade of its AAA credit rating, saying there was a $2 trillion dollar error in the agency's calculations.

"A judgment flawed by a 2 trillion dollar error speaks for itself," a Treasury spokesman said, just after the US lost its AAA rating for the first time ever and was downgraded to a AA+.

It was the first time the US was downgraded since it first received a triple-AAA rating from Moody's in 1917; it has held the S&P rating since 1941.

Moody's and a third ratings agency, Fitch, say they continue to study the deficit plan to see if the US merits being kept in their ranks of AAA countries.

Earlier, an official close to the discussions with S&P said: "There are deep and fundamental flaws with the S&P analysis."