Media playback is unsupported on your device Media caption Timothy Geithner says "S&P has shown really terrible judgement"

The US is unlikely to see its long-term credit rating return to AAA any time soon, ratings agency S&P has said.

Its comments came as it downgraded the state-backed mortgage giants Fannie Mae and Freddie Mac because of their "direct reliance on the US government".

It also lowered ratings for clearing houses and other institutions linked to long-term US debt from AAA to AA+.

Late on Friday, the agency downgraded the US's top-notch AAA rating for the first time in its history.

But US Treasury Secretary Timothy Geithner criticised the decision.

"I think S&P has shown really terrible judgement and they've handled themselves very poorly, and they've shown a stunning lack of knowledge about basic US fiscal math," Mr Geithner told NBC on Sunday.

"I think they drew exactly the wrong conclusion."

'Polarised views'

Media playback is unsupported on your device Media caption David Beers says the US was on "an unsustainable path"

In a conference call on Monday, David Beers, S&P's head of sovereign ratings, said: "We still think that the risks to the rating are still pitched to the downside - that's why we have a negative outlook. We don't anticipate a scenario at the moment in which the US would quickly return to AAA."

He went on: "If [there were] broader consensus among parties about how to make fiscal policy choices over the medium-term horizon and in turn that translated into a more substantial and more robust fiscal stimulation package, those two things could together in time lead to the rating returning to AAA.

"But given the nature of the debate currently in the country, with the polarisation of views across the country right now, we don't see anything immediately on the horizon that would make this the most likely scenario."

US markets were lower on Monday in the first trading session after S&P downgraded the US economy.

The Dow Jones was down 2.4%, while the Nasdaq fell 3.2%.