WASHINGTON (AP) — Alarmed by the chronically weak U.S. economy, the Federal Reserve launched an aggressive new effort Thursday to boost the stock market and make borrowing cheaper for years to come.

And it made clear it won’t stop there and is ready to try other stimulative measures if hiring doesn’t pick up.

Stock prices rocketed up in approval. But economists said the Fed’s plans to buy mortgage bonds for as long as it deems necessary and to keep interest rates at record lows until mid-2015 — six months longer than previously planned — might provide little benefit to the economy.

Chairman Ben Bernanke himself cautioned that the Fed’s actions are no panacea for slow growth and high unemployment, and said the economy will probably need help even after the recovery strengthens.

“The idea is to quicken the recovery,” Bernanke said at a news conference after the Fed lowered its outlook for growth this year.

As part of its bold and open-ended plan, the Fed said it would spend $40 billion a month to buy mortgage bonds to make home buying more affordable. That will be the third round of bond-buying in an effort to spur the economy, and the Fed left open the possibility of taking other steps to encourage borrowing and financial risk-taking.

Stock prices rose steadily after the Fed’s announcement at 12:30 p.m. Eastern time. The Dow Jones industrial average closed up more than 200 points, coming within 625 points — or 4.6 percent — of its all-time high. Other stock averages also surged.