The number of contracts to purchase previously owned U.S. homes jumped in May by the most in more than four years, a sign the residential-real estate market is rebounding after a slow start to the year.

The pending home sales index climbed 6.1 percent, the biggest advance since April 2010, after a revised 0.5 percent increase in April, the National Association of Realtors said Monday in Washington. The gain exceeded the most optimistic estimate in a Bloomberg survey of economists, whose median forecast called for a 1.5 percent gain.