* Fed launches third big round of stimulus until jobs outlook better * Treasury sells $13 billion in 30-year bonds * U.S. jobless claims rose more than expected due to storm By Richard Leong and Luciana Lopez NEW YORK, Sept 13 (Reuters) - Prices for 30-year U.S. government debt sank on Thursday after the Federal Reserve announced a round of stimulus comprising mortgage-backed security purchases rather than the Treasury buys many had expected. But prices for shorter-dated debt rose in volatile trading as investors saw the program, aimed squarely at boosting the housing market in a bid to improve the economy and lower stubborn unemployment, keeping rates lower in the shorter term. The Fed said it would buy $40 billion of mortgage debt per month and keep buying assets until the jobs outlook improves substantially. In an additional move that reflects just how concerned they are about the economy, Fed officials said they were unlikely to raise interest rates from current rock-bottom lows until at least mid-2015, compared with previous guidance of late 2014. The Fed statement initially sent prices for 10- and 30-year debt sharply down, but the benchmark 10-year Treasury note recovered to end the day higher. The 30-year bond dropped, but ended off the day's lows. "The bottom line is for the long end of the Treasury market there was disappointment because they're not extending the buying into the long end. It's a mortgage event," said David Ader, head of government bond strategy at CRT Capital Group in Stamford, Connecticut. Some analysts said the program was less than the market had expected from a third round of quantitative easing, or QE3. "The Fed has under-delivered here. This not full-blown QE3. They have a lot of flexibility with this statement," said Anthony Valeri, a fixed income Strategist at LPL Financial in San Diego. The U.S. Treasury Department sold $13 billion of 30-year bonds earlier in the day, in what analysts said was a strong auction. Fed policymakers are trying to convey "that we're not going to (be) premature in removing policy accommodation, even after the economy starts to recover more quickly, even after the unemployment rate begins to move down more decisively we're not going to rush to begin to tighten policy," Fed Chairman Ben Bernanke said on Thursday after the announcement. "We're going to give it some time to make sure the recovery is well established," he added. Thirty-year bonds traded 19/32 lower to yield 2.950 percent, after the yield briefly pierced 3 percent for the first time in about four months. Ten-year debt ended the day 6/32 higher to yield 1.739 percent, despite having fallen after the Fed announcement. The latest figures on U.S. jobless claims and producer prices did not cause much market reaction earlier in the day. The data largely reinforced the view of low employment growth and inflation. First-time filings for jobless benefits totaled 382,000 last week, higher than what economists had forecast. The Labor Department blamed the larger-than-expected weekly increase on Tropical Storm Isaac. At the same time, the agency said producer prices rose 1.7 percent in August, the biggest monthly increase since June 2009. But the core rate, which excludes volatile energy and food prices, grew 0.2 percent, in line with estimates.