* Yellen's Jackson Hole speech on Friday in focus * Treasury to sell $88 bln new supply By Karen Brettell NEW YORK, Aug 22 (Reuters) - U.S. Treasury prices gained slightly on Monday as investors waited on a speech by Federal Reserve Chair Janet Yellen on Friday for any new indications on when the U.S. central bank will next raise interest rates. Yellen is in focus with no major U.S. economic data due until Friday's second read of gross domestic product for the second quarter. Fed Vice Chairman Stanley Fischer said on Sunday that the Fed is close to hitting its targets for full employment and 2 percent inflation, joining other senior Fed speakers including New York Fed President William Dudley in offering an upbeat view of the U.S. economy in recent weeks. "This morning people are focused on Fischer's remarks," said Gennadiy Goldberg, an interest rate strategist at TD Securities in New York. "The fact that he started off with the words that we are close to our (inflation) target is definitely hawkish," Goldberg said. "The question is does he represent the Fed's view." Treasury yields rose in overnight trading on Fischer's comments, before falling back in the U.S. trading session. Benchmark 10-year notes were last up 8/32 in price to yield 1.55 percent, down from 1.58 percent late on Friday. Dudley last week said that the Fed could raise rates as soon as September, though most economists and investors view a hike that month as a long shot and see December as the most likely month for an interest rate increase. Yellen will speak as central bankers from around the world gather at an annual meeting in the mountains of Jackson Hole, Wyoming. On the supply front, the Treasury Department will sell $88 billion in short and intermediate-dated coupon-bearing debt this week, starting with a $26 billion auction of two-year notes on Tuesday. It will also sell $34 billion in five-year notes on Wednesday and $28 billion in seven-year notes on Thursday.