* Yields rise from one-month lows; Fed uncertainty remains * Treasury sells $13 bln in 10-year TIPS reopening * Treasury to sell $97 bln 2-, 5-, 7-year notes next week * Fed to purchase $1.25 bln-$1.75 bln bonds due 2036-2043 Friday By Karen Brettell NEW YORK, Sept 19 (Reuters) - U.S. Treasuries yields rose on Thursday from one-month lows as investors questioned when the Federal Reserve is likely to begin paring its bond purchases, a day after the U.S. central bank shocked markets by keeping its $85 billion a month bond buying program unchanged. Some profit-taking from Wednesday's rally and hedging of corporate bond issuance weighed on Treasuries on Thursday. At the same time, economic data pointed to an improving economy. Factory activity in the U.S. mid-Atlantic region soared to a 2-1/2-year high in September and firms' optimism about the near future hit a 10-year high, a survey showed on Thursday. But confusion over Fed policy remained the primary factor on investors' minds. The Fed, citing strains in the economy from tight fiscal policy and higher mortgage rates, decided against the tapering of asset purchases that investors had all but priced into stock and bond markets. "The uncertainty does keep a bid in the market, but for now guys are trying to figure out where yields should actually be," said Sean Murphy, a Treasuries trader at Societe Generale in New York. Benchmark 10-year notes were last down 15/32 in price to yield 2.75 percent, up from 2.69 percent late on Wednesday. Five-year notes fell 7/32 in price to yield 1.48 percent, up from 1.43 percent, and 30-year bonds dropped 1-1/32 in price to yield 3.81 percent, up from 3.75 percent. The Treasury also saw relatively soft demand for a $13 billion reopening of Treasury Inflation-Protected Securities (TIPS) on Thursday. The auction came after the bonds had rallied on expectations of a more dovish Fed, which began after Lawrence Summers on Sunday withdrew from consideration to replace Ben Bernanke as Fed Chairman. "Underlying demand for the auction was a little bit light, which is not surprising given all the buying we've been seeing already over the past week. There was a lack of setup," said Michael Pond, head of global inflation-linked research at Barclays in New York. The new debt sold at a high yield of 0.50 percent, around 3 basis points higher than it had traded before the auction. The bid-to-cover ratio of the auction was 2.38 times. Investors also contemplated whether concern by Fed members over still-low inflation may have been a determining factor behind Wednesday's decision to continue its stimulus. Chicago Fed President Charles Evans said earlier this month that he wants to see higher inflation before paring the bond purchase program. St. Louis President James Bullard has expressed similar concerns. "Perhaps that view got a stronger voice at the meeting," said Pond. The Fed on Thursday bought $3.296 billion in notes due 2020 to 2023 as part of its ongoing purchase program. It will buy between $1.25 billion and $1.75 billion in bonds due from 2036 to 2043 on Friday. The Treasury also said on Thursday it will sell $97 billion in new two-year, five-year and seven-year debt next week.