* Treasury auctions $13 billion of 30-year bonds * U.S. weekly jobless claims lower than expected * Investors mull impact of new Fed stimulus By Chris Reese and Luciana Lopez NEW YORK, Dec 13 (Reuters) - U.S. debt prices slid on Thursday after an auction of 30-year bonds saw cautious bidding in the wake of U.S. Federal Reserve monetary policy easing the previous day, with better-than-expected jobs data also eroding Treasuries' safe haven appeal. The Treasury sold $13 billion of 30-year bonds at a high yield of 2.917 percent, and yields rose in the open market for Treasuries for a third straight session. "It would be safe to characterize it as a mixed result," said Ian Lyngen, senior government bond strategist at CRT Capital Group in Stamford, Connecticut. "The street didn't want to participate." U.S. 30-year bond prices rose to session highs shortly before the auction, making it harder for dealers to position themselves to buy the new debt cheaper at the auction. This run-up in prices reduced the bidding for the new 30-year issue at the time of the auction, causing the yield to "tail" or sell at a higher-than-expected level. But 30-year debt gave up those early gains to turn negative after the sale concluded, off 8/32 to yield 2.908 percent. "It's just unfortunately not as stellar an auction" as other recent sales, said William O'Donnell, head of U.S. Treasury strategy at RBS Securities in Stamford, Connecticut. "The tail indicates that where people wanted to buy was close to the cheapest levels yesterday, not where we were," he added. Prices were also undermined by data showing claims for U.S. unemployment benefits were lower than expected in the latest week, eating away at appetite for Treasuries as a safe haven. Investors were still digesting a new round of monetary stimulus announced by the Fed on Wednesday, with the bank shifting more of its purchases to the five-year sector. The Fed committed to monthly purchases of $45 billion in Treasuries on top of the $40 billion per month in mortgage-backed bonds it started buying in September. It will expand purchases to five-year notes from the current seven-, 10- and 30-year Treasuries. Thirty-year bonds particularly underperformed in the wake of the announcement, which also boosted hopes that the economy could strengthen and further bite into the attraction of safe-haven investments. The benchmark 10-year Treasury note traded down 9/32 to yield 1.733 percent, marking the highest yield in more than a month and up from a high yield of 1.65 percent in an auction of $21 billion of the notes on Wednesday. Analysts said the drop in Treasuries was limited by concerns over whether U.S. lawmakers will agree on a deal to avoid a $600 billion mix of spending reductions and expiring tax cuts set to begin in 2013. Such worries were underscored on Wednesday by Fed Chairman Ben Bernanke, who warned that running over this "fiscal cliff" would lead to a new recession. He told reporters the Fed could ramp up its bond buying "a bit" but emphasized that monetary policy has limits and could not fully offset the impact. The Treasury sold $32 billion of three-year notes on Tuesday and $21 billion of 10-year notes on Wednesday. Next week it will sell two-year, five-year and seven-year notes as well as five-year Treasury inflation-protected securities.