WASHINGTON, April 18 (Xinhua) -- The U.S. Treasury Department asserted Monday that the negative credit outlook of Standard and Poor's (S&P) belittled the ability of U.S. leaders to address the difficult fiscal challenges facing the nation, but economists held that the surging public debt would continue to be a grave uncertainty overhang for the world's largest economy.

"This morning, S&P affirmed the AAA rating of the United States, but emphasized the importance of timely bipartisan cooperation and action on fiscal reform," Mary Miller, Assistant Secretary for Financial Markets, said in a statement.

S&P has lowered the long-term U.S. credit outlook from "Stable" to "Negative". It reaffirmed its investment-grade credit ratings on the U.S. short and long-term debt, but said the ratings were at risk in coming years due to spiking deficit and public debt.