By MARTIN CRUTSINGERWASHINGTON (AP) â¿¿ The Federal Reserve isn't yet convinced that the U.S. economy's growth can accelerate without the Fed's drive to keep borrowing costs at record lows. It wants to see sustained improvement. That was the message Fed officials sent Wednesday, when they reinforced their plan to keep short-term interest rates at record lows at least until unemployment falls to 6.5 percent. An unemployment rate of 6.5 percent is a threshold, not a "trigger," for a possible rate increase, Chairman Ben Bernanke said at a news conference. "We are seeing improvement," he said. "One thing we would need is to see this is not temporary improvement." The Fed will continue buying $85 billion a month in bonds indefinitely to keep long-term borrowing costs down. Bernanke said the Fed might vary the size of its monthly purchases depending on whether or how much the job market improves. The unemployment rate has fallen to a four-year low of 7.7 percent, among many signs of a healthier economy. Investors seemed pleased with the Fed's decision to maintain its low-interest rate policies indefinitely for now. The Dow Jones industrial average close up about 56 points, having risen slightly after the Fed's statement was released at 2 p.m. EDT. The Fed's statement took note of the global stresses that have been intensified by the turmoil in Cyprus, which is trying to stave off financial ruin. No longer does the Fed statement say, as it did in January, that "strains in global financial markets have eased somewhat." Bernanke was asked at his news conference whether the flare-up in Cyprus signals that the U.S. financial system might be more vulnerable than bank "stress tests" have shown. He sought to downplay the dangers posed by the tiny Mediterranean nation. Bernanke said that "at this point," he sees no major risks to the U.S. financial system or economy.