The U.K. and the U.S. are at risk of having their top-grade credit ratings downgraded, Moody's warned Monday. Of the four largest economies with AAA ratings, the U.K. and the U.S. are in a "tougher position" than France or Germany, the ratings agency said.

The AAA-rated countries should take strong measures to balance their finances, Moody's recommended, but they must be careful not to hamper economic growth by ending stimulus policies too soon.

The warning comes as the two countries' national debts and the interest rates on them are rising. Moody's predicted the U.S.' ratio of interest payments to government revenue might top 10 percent in 2013, and possibly hit 14 percent in the worst case.

Fitch and Standard & Poor's also warned the U.S. of a credit rating downgrade earlier this year, while S&P downgraded the U.K.'s sovereign rating from "stable" to "negative."