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"There are longer-term pressures on the rating, that's very clear," said Steven Hess, lead analyst for the U.S. credit ratings agency.

Moody's has a stable outlook on the U.S. rating, which indicates a change is not expected over the next 18 months.

"Of course we are continuing to watch U.S. government finances and there are some long-term threats to the U.S. government's financial position that we have to evaluate, but we don't see anything immediate," Hess added.

U.S. stock and bond markets sold off on Thursday amid renewed concerns about the risk of the United States losing its top rating after Standard & Poor's revised its outlook on Britain to negative from stable, indicating the risk of a downgrade.

Asked on Thursday if the U.S. rating was at risk, S&P cited its January affirmation of the AAA rating.

Bill Gross, co-chief investment officer of bond giant Pacific Investment Management Co, said he believes the United States will be downgraded in three to four years.

"Of course the financial strength of the U.S. government is being tested during this crisis and recession period," said Hess.

Whether the rating is eventually threatened will depend on whether debt continues to climb after the recession, he said.

"I think it's too early to answer that question," Hess added.