NEW YORK, Aug 26 (Reuters) - Wall Street lost ground on Thursday, ending a streak of all-time closing highs on concerns over developments in Afghanistan, while fears of a potential shift in U.S. Federal Reserve policy prompted a broad but shallow sell-off the day before the Jackson Hole Symposium. All three major U.S. stock indexes ended the session modestly lower, with the S&P and the Nasdaq notching their first down day in six. The sell-off firmed after hawkish commentary from Dallas Fed President Robert Kaplan and a blast outside the Kabul airport in Afghanistan helped strengthen the risk-off sentiment. read more Kaplan, who is not currently a voting member of the Federal Open Markets Committee, said he believes the progress of economic recovery warrants tapering of the Fed"s asset purchases to commence in October or shortly thereafter. Kaplan"s remarks followed earlier comments from the St. Louis Fed President James Bullard, who said that the central bank is "coalescing" around a plan to begin tapering process. read more "(Kaplan’s statements) caused a little confusion about the taper timeline, but in my opinion the equity markets are focused on geopolitical issues," said Megan Horneman, director of portfolio strategy at Verdence Capital Advisors in Hunt Valley, Maryland. "There’s a flight to safety during geopolitical tensions." "I am surprised the market the market hasn’t fallen more, given the fear that it could take focus away from (U.S. President Joe Biden"s) domestic agenda," Horneman added. The economy grew at a slightly faster pace than originally reported in the second quarter, fully recovering its losses from the most abrupt downturn in U.S. history, according to the Commerce Department. But jobless claims, though still on a downward trajectory, ticked higher last week. read more For an interactive GDP graphic, click here. The data did little to move the needle with respect to expectations that the Fed is unlikely tip its hand regarding the taper timeline when Chairman Jerome Powell unmutes and delivers his speech at Friday"s virtual Jackson Hole Symposium. "We’re going to see a lot of market participants analyze every word (Powell) uses, but at the end of the day, they will begin tapering," Horneman said. "I’m more concerned about the speed at which they taper. What are they going to start with? That will give us a clearer indication as whether they’re getting more hawkish." Unofficially, the Dow Jones Industrial Average (.DJI) fell 191.2 points, or 0.54%, to 35,214.3, the S&P 500 (.SPX) lost 26.27 points, or 0.58%, to 4,469.92 and the Nasdaq Composite (.IXIC) dropped 96.88 points, or 0.64%, to 14,944.98. Discount retailers Dollar General Corp (DG.N) and Dollar Tree Inc (DLTR.O) tumbled after warning higher transportation costs will hurt their bottom lines. read more Coty Inc (COTY.N) jumped after the cosmetics firm said it expects to post full-year sales growth for the first time in three years. read more Salesforce.com Inc (CRM.N) hiked its earnings forecast as the shift to a hybrid work model is expected to fuel strong demand, sending its shares higher. read more NetApp Inc (NTAP.O) surged as brokerages raised their price targets in the wake of the cloud computing firm"s better-than-expected 2022 earnings outlook.