The Federal Reserve has kicked off a much-awaited lending program aimed at jump-starting the $1.45 trillion commercial-paper market, but investors say it could take days or weeks before short-term financing for U.S. companies loosens up.

Under its new Commercial Paper Funding Facility, the Fed is offering to lend money to highly rated companies for as long as three months. The goals are to persuade investors to lend to top-tier companies and give borrowers a backstop if funds can't be obtained in the open market.

The program's impact was muted Monday. Fewer companies came to market looking for financing than last week, and most were limited to uncomfortably short overnight loans. Rates rose modestly for debt maturing in 30 days.

"It will be a few more days before we have a good idea on the impact," said Ira Jersey, interest-rate strategist at Credit Suisse.

The test will be whether rates established in the commercial-paper market are lower than the somewhat punitive rates on the Fed's loans, which are intended to be a source of financing in emergencies rather than the first stop for companies seeking funds.