Wall Street slips after comments by top Fed officials

U.S. stocks ended modestly lower after a volatile session on Friday, having bounced between gains and losses as investors wrestled with the likely timing of a US interest rate hike following comments from top Federal Reserve officials.

The S&P 500 rose after Fed Chair Janet Yellen said the case for raising rates had strengthened but did not indicate when the Fed would act.

At a gathering of central bankers from around the world in Jackson Hole, Wyoming, Yellen said the US economy was nearing the central bank's goals of maximum employment and price stability but that future hikes should be gradual.

Stocks later traded lower after hawkish comments from Fed Vice Chair Stanley Fischer raised the possibility of a rate hike as soon as next month. The S&P 500 rose as much as 0.7 percent and declined by as much as 0.6 percent during the session.