WASHINGTON — The chairman of the Federal Reserve urged banks and regulators on Monday to help the nation’s small businesses get the loans they needed to create jobs, reported The New York Times.

He also acknowledged that economists could not agree on why such lending has contracted substantially over the last two years.

Small businesses — those having fewer than 500 employees — employ half of all Americans and account for about 60 percent of gross job creation. Federal data indicate that lending to such companies fell to below $670 billion in the first quarter of this year from more than $710 billion in the second quarter of 2008.

The reasons are unclear. Many entrepreneurs say that bank loan officers are denying loans to creditworthy borrowers as part of an overreaction to the bad loans of the last economic expansion and heightened scrutiny by regulators.

But several economists paint a more nuanced picture, arguing that weak economic fundamentals and battered balance sheets have lowered the appetite for new lending. They say that demand could take years to recover.

The Fed chairman, Ben S. Bernanke, acknowledged the uncertainty at the start of a daylong forum on small-business lending at the central bank’s headquarters.

“How much of this reduction has been driven by weaker demand for loans from small businesses, how much by a deterioration in the financial condition of small businesses during the economic downturn, and how much by restricted credit availability?” Mr. Bernanke asked. “No doubt all three factors have played a role.”

In a broad outreach effort, the Fed held 43 meetings on the financing needs of small businesses, starting on Feb. 3 in Lexington, Ky., and ending on June 30 in Shreveport, La. Two of the meetings, in Miami and Davenport, Iowa, focused on Hispanic-owned businesses. One, in Denver, was centered on the Small Business Administration’s guaranteed-loan programs. At yet another, in Detroit, the challenges facing auto industry suppliers took center stage.

A collapse in the value of real estate and other collateral used to secure loans posed a “particularly severe challenge” to small businesses, Mr. Bernanke said. He recalled that a business owner at the Detroit meeting told him, “If you thought housing had declined in value, take a look at what equipment is worth.”

Some entrepreneurs have resorted to borrowing on their personal credit cards or from their retirement accounts, he noted.

Banks, for their part, say that they have not so much tightened credit as returned to more traditional underwriting standards after being too lax, Mr. Bernanke acknowledged.

“But, though some lenders said they were emphasizing cash flow and relying less on collateral values in evaluating creditworthiness,” Mr. Bernanke said, “it seems clear that some creditworthy businesses — including some whose collateral has lost value but whose cash flows remain strong — have had difficulty obtaining the credit that they need to expand, and in some cases, even to continue operating.”