The United States faces a one-in-four chance of slipping back into recession, a Reuters poll showed.

A weaker economic outlook is raising the likelihood the Federal Reserve will soon do more to boost growth. Just two weeks ago, economists estimated the chances of another recession at one-in-five.

The poll, taken as world stock markets staged their biggest fall since the dark days of the financial crisis in 2008, showed lower growth expectations for the rest of this year.

U.S. President Barack Obama said Monday that the real economic challenge for U.S. policymakers is to take effective steps to improve the long- term fiscal sustainability. President Obama said this after Standard & Poor's (S&P) stripped the nation of its top-notch credit rating Friday.