WASHINGTON – 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.