WASHINGTON, May 15 (Reuters) - Average interest rates on U.S. 30- and 15-year mortgages fell to new record lows in the week ending May 16, Freddie Mac said on Thursday. Thirty-year mortgage rates fell to an average of 5.45 percent -- the lowest on Freddie Mac records dating back to 1971 -- from 5.62 percent last week. Fifteen-year mortgages dipped to an average of 4.84 percent -- the lowest also on Freddie Mac records going back to 1991 -- from 4.97 percent last week. One-year adjustable rate mortgages (ARM) stood at an average of 3.67 percent, almost unchanged from a record low average of 3.66 percent the prior week. A year ago, 30-year mortgages stood at an average of 6.89 percent, 15-year mortgages at 6.37 percent and the ARM at 4.81 percent. "Continued reaction to last week's Federal Reserve Committee statements about the threat of deflation has triggered a rally in the bond market, driving long-term yields to the lowest level since 1958," Freddie Mac chief economist Frank Nothaft said in a statement. "When that happens, mortgage rates are bound to drop and indeed, that is just what they did this week," he said. The Fed last week kept U.S. interest rates steady at 1961 lows but warned of a possible "unwelcome substantial fall in inflation," saying this meant the economy was in danger of weakness ahead. Earlier on Thursday the Labor Department said U.S. April wholesale prices fell 1.9 percent, the largest one-month drop on record. Nothaft said the report reaffirmed concerns about the state of the economy.