LONDON (AP) — The United States could lose its top credit rating from a leading agency for the second time if there's a delay in raising the country's debt ceiling, Fitch Ratings warned Tuesday.

Congress has to increase the country's debt limit, which effectively rules how much debt the U.S. can have, by March 1 or face a potential default. There are fears that the debate will descend into the sort of squabbling and political brinkmanship that marked the last effort to raise the ceiling in the summer of 2011. The U.S. Treasury Department warned then that it had nearly reached a point where it would be unable "to meet our commitments securely."

Standard & Poor's was so concerned by the dysfunctional nature of the 2011 debate that it stripped the U.S. of its triple A rating for the first time in the country's history. Like Fitch, Moody's has a negative view on the U.S. outlook.

"The pressure on the U.S. rating, if anything, is increasing," said David Riley, managing director of Fitch Ratings' global sovereigns division. "We thought the 2011 crisis was a one-off event .... if we have a repeat we will place the U.S. rating under review."

Fitch already has a negative outlook on the U.S. as the country's debt burden has risen to around 100 percent of its gross domestic product, and has said it will make a decision on the rating this year, regardless of how the debt ceiling discussions pan out. The U.S. government reached its statutory debt limit of nearly $16.4 trillion at the end of 2012 but has engineered extraordinary measures that should see it through February.

Riley's comments come just two weeks after U.S. lawmakers agreed a budget deal with the White House that avoided the so-called "fiscal cliff" of automatic tax increases and spending cuts that many economists thought could plunge the U.S. economy, the world's largest, back into recession. Relief that a deal was cobbled together, albeit at the final hour, is one of the reasons why sentiment in the financial markets has been buoyant in the first trading days of the new year. Many stock indexes around the world are trading at multi-year highs.

"The 'fiscal cliff' bullet was dodged .... (but it's) a short-term patch," said Riley.

Riley warned that the different arms of the U.S. government still have a number of issues to address. As well as increasing the debt ceiling, they have to agree to spending cuts that were delayed as part of the 'fiscal cliff' agreement and back measures to avoid a government shutdown, potentially in March.

Though short-term fixes are more likely than not, Riley said the U.S. political environment is not as good as it should be for a country holding the gold-chip AAA rating. The past few years, Riley said, have been marked by "self-inflicted crises" between deadlines.

The major reason behind the lack of swift action in the U.S. is that the Democrats control the White House and the Senate, while the Republicans have a solid majority in the House of Representatives. Both sides have differing visions of the role of the state in society and often varying political objectives.

Despite his cautious tone on the rating, Riley said the U.S. has a number of huge advantages and that getting the country's public finances into shape will not require the same level of austerity that many countries in Europe have had to enact over the past few years, partly because the U.S. economy is growing at a steady rate.

Other factors Fitch says support the U.S.'s AAA rating are the country's economic dynamism, lower financial sector risks, the rule of law as well as the global benchmark status of the country's bonds and the dollar.

However it says these "fundamental credit strengths are being eroded by the large, albeit steadily declining, structural budget deficit and high and rising public debt."