LONDON—Two leading rating firms have cautioned the U.S. on its credit rating, expressing concern over a deteriorating fiscal situation that they say needs correction.

The warnings issued Thursday echoed prior statements by the companies, however, and financial markets largely ignored them. Treasury yields, which move in the opposite direction as prices, were lower in late-morning trade and the cost of insuring U.S. debt against the risk of default, already below that of Germany, the euro-zone benchmark, barely budged.