Global stock market sentiment had weakened before Standard & Poor's cut the U.S. credit rating to AA+ from AAA, the rating agency's global head of sovereign ratings David Beers said on Friday.

S&P downgraded the United States' prized triple-AAA rating on August 5, exacerbating a rout in global stock markets that had already been hit by Europe's growing sovereign debt crisis and fears of a renewed U.S. recession.

From our perspective, it's an oversimplification to say this was happening because of S&P's downgrade, Beers said, referring to criticism that the move caused volatility in the market.

World stocks as measured by MSCI's All-Country World Index have fallen about 17 percent from their May high as markets lose faith with the ability of politicians on both sides of the Atlantic to tackle mounting debt burdens.

Many investors are pessimistic about the outlook for rich world economies, particularly the euro zone, where the big fear is that the debt crisis that has swamped Greece, Portugal and Ireland will spread to bigger and much harder to save economies such as Spain or Italy.

We're waiting to see if the governments can get their act together and address both the short-term and long-term issues, Beers told a media roundtable discussion in Singapore, referring to developed countries in general.

S&P, one of the big three ratings agencies, also said that the outlook for Asian sovereigns was stable, but was showing some downside risks.

Most Asian countries, especially those such as Singapore, South Korea or Taiwan with a higher share of exports to Western countries, would be hurt if the United States or European economies continued to slow, said Elena Okorotchenko, managing director at S&P.

(Reporting by Kevin Lim and Charmian Kok; Writing by Saeed Azhar and Alex Richardson; Editing by Kim Coghill)