Stocks declined Monday, with the Standard & Poor's 500 index posting its worst month in three years, as investors worried over slowing global growth and the impact of a potential interest-rate increase by the Federal Reserve as soon as September.

The S&P 500 turned in its biggest monthly slide since May 2012. The Dow Jones industrial average sank to complete its worst monthly drop since May 2010.

At the close on Wall Street the Dow had given up nearly 115 points, about 0.7 percent, to 16,528. The S&P 500 was down 16.7 points, about 0.8 percent, to 1,972.2. The Nasdaq composite declined 51.8 points, about 1.1 percent, to 4,776.5.

"There's so much emotion right now, and in this environment you can come in any morning and have something out of Europe or Asia crossing us and that's what causes us to move," said Steve Bombardiere, an equity trader at Conifer Securities in Manhattan. "There were a lot of people who wanted to buy a correction, but after last week they paused and are thinking about how long it is going to last."

CRUDE ENERGY: The price of benchmark U.S. crude reversed earlier losses, and as markets closed was up $3.24 cents to $48.46 a barrel on the New York Mercantile Exchange.

FED PLANS: Fed vice chairman Stanley Fischer said over the weekend that policymakers still had a "pretty strong case" for raising rates in September. That ran counter to recent market sentiment that China's economic slowdown and global market volatility might prompt the nation's central bank to wait.

Speaking at the Fed's annual gathering in Jackson Hole, Wyoming, Fischer emphasized he was not saying what action Fed policymakers might take at the September meeting of the Open Market Committee, but analysts took his comments to mean he saw the economy moving close to satisfying the Fed's conditions for an interest rate hike. The Fed has kept rates ultra-low since the 2008 financial crisis.

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GO AWAY AUGUST: It's been a brutal month for investors. On the last the last trading day of August, the S&P 500 is down 6.2 percent for the month, its worst loss since May 2012. Last week the S&P 500 index briefly entered a "correction," defined as a drop of 10 percent or more from a recent peak.

Volatility has also risen sharply. The VIX, Wall Street's so-called fear gauge, soared 132 percent this month, the largest monthly jump the index has ever had.

Reports from The Associated Press and Bloomberg News were used in this story.