SAN FRANCISCO (MarketWatch) -- Moody's Investors Service said late Wednesday it placed the U.S. government's triple-A bond rating on review for possible downgrade due to rising risk of default. "The review of the U.S. government's bond rating is prompted by the possibility that the debt limit will not be raised in time to prevent a missed payment of interest or principal on outstanding bonds and notes. As such, there is a small but rising risk of a short-lived default," Moody's said. The rating agency stressed that it considers the probability of a default to be low but no longer minimal. "An actual default, regardless of duration, would fundamentally alter Moody's assessment of the timeliness of future payments, and a Aaa rating would likely no longer be appropriate," it added. Moody's said the rating, if lowered, would most likely be somewhere in the Aa range.