U.S. 30-year fixed mortgage rates dropped to 3.78 percent in the week ending May 24, down slightly from 3.79 percent in the previous week, mortgage financier Freddie Mac said Thursday.

This marks the fourth straight week the rate has dropped to a record low, and it's far below the 4.60 percent interest rate reported during the same period last year.

Mortgage rates were virtually unchanged this week with fixed-rate loans remaining at record lows and helping to drive homebuyer affordability, said Frank Nothaft, Freddie Mac's chief economist.

The 15-year rate was unchanged at 3.04 percent while the five-year Treasury-indexed adjustable-rate mortgage rate was flat at 2.83 percent. One-year ARMs were down to 2.75 percent from last week's 2.78 percent.

The downward trend in mortgage rates has sent the National Association of Realtor's Housing Affordability Index to its highest level since its inception in 1970.