(Updates to add Freddie Mac official's comment on housing sales, Lockhart on commercial real estate risk)

By Ros Krasny and Kristina Cooke

NASHVILLE, Tenn, April 18 (Reuters) - Top U.S. officials on Saturday offered reassurances that the worst of the economic downturn is likely over, helped by unprecedented efforts to keep credit flowing, though the recovery will be slow.

Two Federal Reserve policy-makers, Vice Chairman Donald Kohn and New York Fed chief William Dudley, both pointed to signs that measures taken by the U.S. central bank are indeed working to help revive the economy.

And Paul Volcker, a senior economic adviser to the Obama administration and a former Fed chairman himself, said the rate of the economy's decline is set to slow.

Volcker, who like the other officials spoke at a conference of policy-makers and academics at Vanderbilt University, said, however, that the economy faces a "long slog" toward recovery.

Fed officials often stress that there is a lag before the economy responds to measures taken to support growth, such as interest rate cuts, and that those lag times can vary.

Since the financial crisis erupted in 2007 the Fed has slashed its benchmark lending rate from a peak of 5.25 percent, reaching the current range of zero to 0.25 percent in December 2008.

The Fed has also created an alphabet-soup of programs to support credit markets and revive lending in different segments, especially home mortgages.