U.S. stocks closed mixed in choppy trading Monday, amid continued concerns that the Federal Reserve may scale down its bond purchases earlier than expected. The Dow Jones Industrial Average inched down 9.53 points, or 0. 06 percent, to 15,238.59 points. The Standard & Poor\'s 500-stock Index lost 0.57 points, or 0.03 percent, to 1,642.81 points. The Nasdaq Composite Index added 4.55 points, or 0.13 percent, to 3, 473.77 points. The main stock indexes opened higher after Standard & Poor\'s Ratings Services upgraded U.S. credit outlook to stable from negative. \"Our sovereign credit ratings on the U.S. primarily reflect our view of the strengths of the U.S. economy and monetary system, as well as the U.S. dollar\'s status as the world\'s key reserve currency,\"the rating agency said in a news release Monday.\"We do not see material risks to our favorable view of the flexibility and efficacy of U.S. monetary policy. We believe the U.S. economic performance will match or exceed its peers\' in the coming years. We forecast that the external position of the U.S. on a flow basis will not deteriorate.\" The market was also underpinned by a strong performance in the stock market in Japan, with the Nikkei 225 Stock Average closing up 4.94 percent Monday, thanks to an upgrade of Japanese gross domestic product for the first quarter and a weak yen. A sharp decline in China\'s exports in May, however, capped Wall Street\'s gains. China\'s exports last month inched up merely 1 percent year on year from a 14.7 percent increase in April, while imports declined 0.3 percent in May from a 16.8 percent gain in the prior month, according to official data released over the weekend. With no major U.S. economic data due out on Monday, investors continued to question when the Federal Reserve may scale back its quantitative easing policy. Federal Reserve Bank of St. Louis President James Bullard said on Monday the U.S. labor market has improved since last summer, suggesting the Fed may slow down the pace of its bond purchases earlier than expected. The remarks added negative market sentiment. The stock market volatility has increased obviously in the past two or three weeks since Federal Reserve Chairman Ben Bernanke said in his testimony before Congress on May 22 that the U.S. central bank could curtail its bond purchases in the next few policy-setting meetings depending on the economic data. The Fed will hold its monetary policy meeting next week. Wall Street surged Friday as a tepid U.S. non-farm payroll report for May fueled investors\' expectation that the Fed could maintain its monetary stimulus for the time being. In corporate news, shares of Apple fell 0.66 percent to 438.89 U.S. dollars though the tech giant unveiled its iTunes radio, a free Internet radio service, at its ongoing annual Worldwide Developers Conference.