WASHINGTON (AP) — The Fitch credit rating agency has warned that it is reviewing the U.S. government's AAA credit rating for a possible downgrade, citing Thursday's looming deadline to increase the nation's borrowing limit.

Fitch has placed the U.S. credit rating on negative watch, a step that would precede an actual downgrade. The agency said it expects to conclude its review within the next six months.

Fitch says it expects the debt limit will be raised soon, but adds, "the political brinkmanship and reduced financing flexibility could increase the risk of a U.S. default."

Fitch is one of the three leading U.S. credit ratings agencies, along with Standard & Poor's and Moody's. S&P downgraded U.S. long-term debt to "AA" in August 2011.