IRVINE, Calif., Nov 5 (Reuters) - The U.S. Federal Reserve's unconventional monetary policies have lowered borrowing costs and boosted growth without creating unwanted side effects like inflation, a top Fed official said on Monday.

Although there have been concerns about the unintended consequences of such policies, "the available evidence suggests they have been effective in stimulating growth without creating an undesirable rise in inflation," John Williams, president of the San Francisco Federal Reserve Bank, said in remarks prepared for delivery at the University of California, Irvine.

The policies also have not stimulated excessive risk-taking, he said. The Fed watches carefully for signs of both inflation and excessive risk-taking and has tools to combat both should they emerge, he said.

The Fed lowered short-term interest rates to zero in December 2008, and has bought more than $2 trillion in long-term securities to lower borrowing costs even more. In August 2011 it moved further into unconventional territory by saying it planned to keep rates ultra-low for about two more years.

In September, the Fed went still further into uncharted territory, launching a third round of asset purchases and promising to keep rates low until at least mid-2015.

Such large-scale asset purchases and the Fed's guidance on how long it will keep interest rates low together have proven effective at helping the economy, Williams said.

Citing research, he said the Fed's first two rounds of asset-buying likely shaved 1.5 percentage points from the unemployment rate. They also likely kept the U.S. economy from falling into deflation, he said.

Quantifying the effects of the Fed's policies is difficult, he added, but "the presence of uncertainty does not mean that we shouldn't be using these tools."

Williams has been a strong supporter of the U.S. central bank's super-easy monetary policy and is a voter this year on the Fed's policy-setting committee.