WASHINGTON — In a surprise that sent the stock market soaring, the Federal Reserve concluded Wednesday that the US economy isn’t yet healthy enough for the central bank to ease its stimulus even slightly.
The cautious message pleased investors, who had expected a slight cut in the bond purchases that have kept long-term interest rates low as the nation recovers from the Great Recession. Wall Street celebrated the prospect of continued low interest rates by lifting stocks to a record high.
In its statement, the Fed signaled it has no set timetable for reducing the stimulus, which has stood at $85 billion a month for the past year.
Chairman Ben Bernanke explained at a news conference that there are good reasons to be cautious:
■ The Fed has yet to see conclusive evidence the job market and economy are approaching full health.
■ Mortgage rates have surged, and the bond purchases are needed to hold those rates down and keep home buying affordable for ordinary people.
■ A budget stalemate in Congress and the threat of a government shutdown as soon as next month are holding back growth and putting the economy at risk.
‘‘Conditions in the job market today are still far from what all of us would like to see,’’ Bernanke said.
Stocks spiked immediately after the Fed released its statement at the end of its two-day policy meeting. The Dow Jones industrial average jumped 147.21 points, or 1 nearly percent.
The Fed’s decision raised hopes for lower rates on bonds and consumer and business loans. Bond yields sank. The yield on the 10-year Treasury note fell to 2.71 percent from 2.85 percent, the biggest one-day drop in nearly two years.
Since May, when Bernanke signaled the Fed could reduce its bond purchases this year, average rates on long-term fixed mortgages have climbed more than a full percentage point to near two-year highs. The average on the 30-year loan is at 4.57 percent, according to Freddie Mac.
There are signs higher mortgage rates have made it harder for people to afford homes at a time when the rebound in the housing market has been a pillar for the economy.
The Fed lowered its economic growth forecasts slightly for this year and next. It predicts the economy will grow just 2 to 2.3 percent this year, down from its forecast in June of 2.3 to 2.6 percent.
Next year’s economic growth will be a barely healthy 3 percent, the Fed predicts.
The Fed’s policy makers expect the unemployment rate to fall to between 7.1 and 7.3 percent by the end of 2013, slightly below its June forecast of 7.2 to 7.3 percent. It predicts unemployment will fall as low as 6.4 percent next year, down from 6.5 percent in its June forecast.
The Fed said rising mortgage rates and government spending cuts are restraining growth. It repeated a plan to keep key short-term rates near zero at least until unemployment falls to 6.5 percent from the current 7.3 percent. The Fed’s short-term rate indirectly affects many consumer and business loans.
‘‘We’re in a slow-growth economy with high unemployment and low inflation,’’ said Greg McBride, senior financial analyst at Bankrate.com. ‘‘There’s no specific catalyst for the Fed to remove stimulus.’’
David Robin, an interest-rate strategist at Newedge LLC, said policy makers were surprised by how fast rates rose after they raised the possibility of scaling back the bond purchases. They probably worried that rates would rise even more, and jeopardize the economy, if they reduced the bond-buying.
Bernanke said the Fed is concerned that looming fights between Congress and the White House over the budget and taxes could slow the economy. Unless Congress agrees to fund the government past Oct. 1, a shutdown will occur.
The government is also expected to reach its borrowing limit next month. Unless Congress agrees to raise the limit, the government won’t be able to pay all its bills.
‘‘This is one of the risks we are looking at,’’ Bernanke said.
The Fed’s policy statement was approved on a 9-to-1 vote. Esther George, president of the Federal Reserve Bank of Kansas City, dissented, for the sixth time this year. She repeated her concerns that the bond purchases could fuel high inflation and financial instability.
The decision to maintain the stimulus follows reports of sluggish economic growth. Hiring slowed this summer, and consumers spent more cautiously.
Economists suggested the Fed will still scale back its bond buying, perhaps before year’s end. ‘‘Tapering will come sooner rather than later, assuming that the economy cooperates,’’ Sung Won Sohn, an economist at California State University Channel Islands, wrote in a research report. ‘‘The economy is steady, though not strong, and is moving in the right direction.
Yet the unemployment rate has dropped in large part because many people have stopped looking for work and are no longer counted jobless.