WASHINGTON — U.S. home prices gains accelerated in April even as sales have stumbled, a sign the coronavirus outbreak has had little impact on real estate values.

The S&P CoreLogic Case-Shiller 20-city home price index climbed 4% in April compared with a year earlier, the largest gain since December 2018, up from 3.9% in March.

Home sales have fallen sharply for three straight months to their lowest annual pace in nearly a decade in May. Yet the supply of available houses for sale has also declined, compared with a year ago, forcing remaining buyers to bid up prices.

“The price trend that was in place pre-pandemic seems so far to be undisturbed, at least at the national level,” said Craig Lazzara, managing director of S&P Dow Jones Industries. “Prices in 12 of the 20 cities in our survey were at an all-time high in April.”

Sales of existing homes are likely to pick up in coming months, however. A measure of signed contracts to buy homes soared 44% in May, a record increase. And sales of new homes also rebounded in May.

Phoenix posted the biggest price gain with an increase of 8.8%, compared with a year earlier. Seattle followed with 7.3% and Minneapolis with 6.4%. Prices from the Detroit metropolitan area were not included in the 20-city index because of delays at the recording office in Wayne County, which includes Detroit.

The Case-Shiller index is composed of a three-month average of home prices, so this month’s data includes figures from February, March and April.