Moody's Investors Service warned on Thursday of the growing likelihood that it could revise the outlook on the AAA credit rating of the United States to negative in the next two years.

Moody's had already said late last year that the extension of Bush-era tax cuts for two more years would add to the likelihood of a negative outlook on the U.S. rating.

In a report issued on Thursday, the agency provided more details about the risks to U.S. ratings.

Recent trends in and the outlook for government financial metrics in particular indicate that the level of risk, while still small, is rising and likely to continue to rise in the next several years, the ratings agency said in a report.