* 10-year yields fall to lowest since July 3 * Bernanke says plan to pare bond purchases is flexible * Fed's timing on phasing out stimulus will depend on data By Karen Brettell NEW YORK, July 17 (Reuters) - Treasuries yields fell to their lowest levels in two weeks on Wednesday after Federal Reserve Chairman Ben Bernanke said plans to pare its bond purchase program are not set in stone, pushing back some expectations of when a scaling back might begin. While sticking closely to a timeline he first announced last month that the Fed would halt bond buying, or quantitative easing, by mid-2014 when unemployment was projected to be around 7 percent, Bernanke went out of his way to stress that nothing was certain. "Our asset purchases depend on economic and financial developments, but they are by no means on a preset course," he told the U.S. House of Representatives Financial Services Committee in prepared remarks. "He sounded a little more downbeat on the economy than the most recent Fed policy statement. Mainly he's trying to drive home his point that Fed policy is data-dependent and that they are in control of any timing and pace of any QE tapering," said Michael Lorizio, senior fixed-income trader at John Hancock Asset Management in Boston. Many economists and investors had expected that the Fed will begin reducing purchases in September, though Bernanke's dovish tone on Wednesday led some to think that it may instead begin next year. "It looks like he's not committed to putting the start of tapering in September.the timeframe for tapering has been pushed out," said Sean Murphy, a Treasuries trader at Societe Generale in New York. Benchmark 10-year Treasuries were last up 13/32 in price to yield 2.49 percent, the lowest level since July 3 and down from 2.55 percent before the comments. Fed officials have tried to talk down fears over the pace in which it will withdraw stimulus after a dramatic rise in bond yields caught many investors by surprise, and increased concerns that the Fed's efforts to exit from its unprecedented stimulus would create even more volatility. Ten-year Treasuries yields reached two-year highs of 2.76 percent on July 8, up more than a full percentage point from the beginning of May. Bernanke has also aimed to soothe markets by emphasizing that the Fed will keep rates low for a long time to come and that it will not sell the debt and will reinvest proceeds in new purchases. The Fed views the size of its bond holdings as having a stimulative impact on the economy by holding yields lower than they would otherwise be, even if it is no longer continuing purchases.