NEW YORK — The Federal Reserve is likely to start a fresh round of unorthodox stimulus today by announcing a plan to purchase at least $500 billion of long-term securities, according to economists surveyed by Bloomberg News.

Policymakers meeting today and Thursday will restart a program of securities purchases to spur growth, reduce unemployment and increase inflation, said 53 of 56 economists surveyed last week. Twenty-nine estimated the Fed will pledge to buy $500 billion or more, while another seven predicted $50 billion to $100 billion in monthly purchases without a specified total. The remainder said the Fed would buy up to $500 billion, or they didn’t quantify their forecast.

The varied responses reflect differences among Fed officials over the total amount of purchases needed to energize the recovery. Policymakers, pursuing unprecedented stimulus, have cut the benchmark rate almost to zero and bought $1.7 trillion in securities without generating growth fast enough to bring down unemployment from near a 26-year high.

“There’s no silver bullet right now,” and central bankers have “very few options left in terms of lowering interest rates,” said John Silvia, chief economist at Wells Fargo Securities in Charlotte, N.C. He predicted $500 billion in Treasury and mortgage-backed securities purchases over the next six months.

New York Fed President William Dudley set expectations at $500 billion in purchases when he said in an Oct. 1 speech that purchases totaling about that amount would add as much stimulus as lowering the Fed’s benchmark rate by half a percentage point to three-quarters of percentage point.

St. Louis Fed President James Bullard said the Fed should buy $100 billion in long-term Treasuries this month and calibrate subsequent purchases based on the course of the economy. Atlanta Fed President Dennis Lockhart said that a pace of $100 billion of purchases a month is “in the range of numbers one might consider.”

Estimates by economists about the duration of a Fed asset-purchase program ranged from as short as three months to as long as the end of 2011.

The lack of clarity over the Fed’s plans has played out in the Treasury market, which handed investors a loss of 0.18 percent in October, the first negative monthly return since March, according to index data compiled by Bank of America Merrill Lynch. After falling to 2.38 percent on Oct. 7 from 2.51 percent on Sept. 30, the yield on 10-year Treasuries has since climbed to 2.63 percent as of late Monday, Bloomberg data show.

Not all Fed officials agree the central bank should start new stimulus measures. Kansas City’s Thomas Hoenig, who has already dissented six straight times, said Oct. 25 that he opposes more easing because it’s “a very dangerous gamble” that may accelerate inflation and create asset-price bubbles. Dallas Fed President Richard Fisher and the Philadelphia Fed’s Charles Plosser have also spoken out against more action by the central bank.

Chicago Fed President Charles Evans said several times last month that the central bank needs to take action and should buy securities on a large scale to carry out his preferred strategy of aiming to raise inflation temporarily.

“They’re in uncharted waters here, and no one really knows, we’re all guessing” about the size and duration of the easing program and its ultimate impact, said Stephen Stanley, chief economist at Pierpont Securities in Stamford, Conn. “I haven’t seen anybody out there who has made a convincing case that this is anything but a trivial boost for the economy.”