One of the three major credit ratings agencies warned Wednesday that a failure to raise the debt ceiling could result in a lower credit rating for the U.S.

Fitch Ratings currently assigns a AAA rating to U.S. debt, the highest level possible. However, a failure to raise the debt ceiling "may not be compatible with 'AAA' status," according to the agency.

If the U.S. cannot sell more debt after bumping up against the debt ceiling, it may not be able to make all of its interest payments on time and in full. The federal government could begin running out of cash as soon as October.

The debt ceiling is currently $19.9 trillion, and Treasury Secretary Steven Mnuchin has repeatedly urged Congress to raise the debt ceiling by September 29. A failure to do so could roil financial markets around the world, and ultimately increase the cost of servicing U.S. debt.

This is not the first time Congress has faced this problem. During an earlier debt ceiling showdown in 2011, Standard & Poor's reduced its rating on U.S. debt from its highest level to AA+. However, Fitch and Moody’s stuck with their top ratings.