WASHINGTON — US home price gains slowed for the ninth straight month in December, reflecting weaker sales and higher mortgage rates that have since declined.

The S&P CoreLogic Case-Shiller 20-city home price index increased 4.2 percent from a year earlier, down from 4.6 percent in November, according to a report released Tuesday.

Home sales and price increases cooled considerably last year and have been a drag on the economy. Previous price gains have put many homes out of reach for would-be buyers, and a jump in mortgage rates last fall also held back sales, which plunged 8.5 percent in 2018.

Prices rose the fastest in Las Vegas, Phoenix and Atlanta. Seattle and Portland, which spent months as the hottest real estate markets nationwide, ranked 11th and 16th in price gains in December.

Washington, D.C. and San Diego saw the smallest cost increases.

Additional evidence of the housing market's troubles emerged Tuesday, when a government report showed that developers started work in December on the fewest new homes in more than two years. That suggests builders are anticipating fewer sales of new homes this year.

The average rate on a 30-year fixed mortgage reached 4.75 percent in early December, nearly a percentage point higher than a year earlier. It has since fallen back to 4.35 percent, which could boost sales a bit this year.

Even as home price increases weaken, they are still increasing faster than average paychecks, which will likely limit sales.