WASHINGTON - The Federal Reserve on Wednesday stood by its efforts to keep borrowing costs at record lows, saying it isn't yet convinced that the U.S. economy's growth can accelerate without significant help from the central bank.

It wants to see sustained improvement.

Fed officials reinforced their plan to keep short-term interest rates at rock-bottom levels 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.

"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 the job market improves and by how much.

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-rate policies for now. The Dow Jones industrial average closed up about 56 points, having risen slightly after the Fed's statement was released.

The Fed's statement took note of the global stresses that have been intensified by 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 a 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.

"At this point," Bernanke said, he sees no major risks to the American financial system or economy.

In its statement, the Fed noted that the U.S. job market has improved, consumer spending and business investment have increased and the housing market has strengthened. But its latest economic forecasts, also released Wednesday, show that the Fed still doesn't expect unemployment to reach 6.5 percent until 2015.

The Fed also cautioned that government spending cuts and tax increases could slow the economy. It predicts that growth won't exceed 2.8 percent this year, slightly lower than its December forecast of 3 percent.

A total of 13 Fed officials still think the first rate increase won't occur until 2015, the same number that thought so in December. One Fed official thinks the first boost in the short-term lending rate won't occur until 2016.

The statement was approved on an 11-1 vote.

Esther George, president of the Kansas City regional Fed bank, dissented for a second straight meeting. She reiterated her concerns that the Fed's aggressive stimulus could heighten the risk of inflation and financial instability - a concern shared by other critics.

Some economists say they fear the Fed has pumped so much money into the financial system that it could eventually ignite inflation, fuel speculative asset bubbles or destabilize markets once the Fed has to start raising rates or unloading its record $3 trillion investment portfolio.

And while the Fed's low interest-rate policies are intended to boost borrowing, spending and stock prices, they also hurt millions of retirees and others who depend on income from savings.

"Things are not going to get better for savers," said Greg McBride, senior financial analyst at Bankrate.com. "Rates are going to stay low for borrowers, and the Fed's accommodation will continue to be a positive for the stock market. Right now, the market is addicted to Fed stimulus."

Jim O'Sullivan, chief U.S. economist at High Frequency Economics, said the Fed appears focused on "whether recent improvement continues, and no changes to the (bond) purchase program appear imminent."

But O'Sullivan said he thinks the Fed might scale back its bond purchases in the second half of this year if job growth continues to accelerate.

Brian Bethune, an economics professor at Gordon College in Wenham, Mass., said the Fed's first move might be to reduce its monthly bond purchases in the October-December quarter of this year and again in the first quarter of 2014. Reducing the Fed's bond purchases would likely cause interest rates to rise, making loans more expensive, and possibly cause stock prices to fall.

The economy slowed to an annual growth rate of just 0.1 percent in the October-December quarter, a near-stall that was due mainly to temporary factors that have largely faded. Economists think growth has rebounded in the January-March quarter to an annual rate around 2 percent or more. The most recent data support that view.

Americans spent more at retailers in February despite higher Social Security taxes that shrank most workers' paychecks. Manufacturing gained solidly in February. And employers have gone on a four-month hiring spree, adding an average of 205,000 jobs a month. In February, the unemployment rate, though still high, reached its lowest point since December 2008.

One reason for the Fed's reluctance to reduce its stimulus is the history of the past three years. In each of the three, economic prospects looked promising as the year began. Yet in each case, the economy stumbled.

Though the economy has brightened this year, it still faces threats, including across-the-board government spending cuts that took effect March 1 and are expected to trigger furloughs and layoffs.

The Fed's forecasts for the economy are rosier than those issued by the Congressional Budget Office. The CBO has warned that the government spending cuts, along with the Social Security tax increase and higher taxes on top earners, could slow growth by 1.5 percentage points this year, to 1.5 percent.

Bernanke, whose second four-term term as chairman ends in January 2014, was asked whether he might stay on for another term. Doing so could allow him to manage the Fed's delicate transition from stimulating the economy to selling its vast investment portfolio and gradually raising interest rates to prevent inflation.

"I don't think I am the only person in the world who can manage the exit," Bernanke said. "With respect to my personal plans, I will certainly let you know when I have something more concrete."