The average U.S. mortgage rate fell below a key threshold of 4 percent this week, its lowest level in five months.

Mortgage buyer Freddie Mac says the average on 30-year fixed-rate home loans declined to 3.97 percent this week from 4.08 percent last week. Interest rates on mortgages began to rise after President Donald Trump won the November election. But they've started falling as the fate of tax reform and other policies has become uncertain.

The 30-year rate stood at 3.59 percent a year ago and averaged 3.65 percent in 2016, the lowest level in records dating to 1971. Lower rates make it easier for homebuyers to afford their monthly mortgage payments.

The rate on 15-year mortgages declined to 3.23 percent from 3.34 percent last week.