Sales of new U.S. single-family homes unexpectedly increased in April as builders lowered prices to lure buyers, but rising mortgage rates and an uncertain economic outlook remain constraints for the housing market.

The slowest sales pace for any April in 16 years indicates the spring selling season is shaping up as a bust.

Sales of previously owned homes in April declined 0.5% from March to a seasonally adjusted, annualized rate of 4 million units, according to the National Association of Realtors. That is the slowest April pace since 2009. Sales were down 2% from April of last year. Housing economists were expecting a gain of 2.7%.