Contracts to buy previously owned U.S. homes surged far more than expected in April to the highest level in more than a decade, another sign the economy has gained steam during the second quarter.

The National Association of Realtors said on Thursday its pending home sales index, based on contracts signed last month, increased 5.1 percent to 116.3, a level not seen since February 2006.

Economists polled by Reuters had forecast pending home sales rising 0.6 percent last month. Contracts usually become sales after a month or two.

The increase in pending home sales for March also was revised marginally upward. Overall pending home sales last month were up 4.6 percent from a year ago.

The housing sector has steadily strengthened on the back of an economy near full employment and historically low mortgage rates.

Contracts rose in three of the nation's four regions, with the West reporting an 11.4 percent jump and the South showing a 6.8 percent increase.

The Midwest reported a 0.6 percent decline, but pending home sales in that region were still up 2.0 percent from a year ago.