Contracts to buy U.S. previously owned homes fell more than expected in April as rising mortgage rates and economic uncertainty weighed on demand.

The National Association of Realtors (NAR) said Thursday its Pending Home Sales Index, based on signed contracts, dropped 6.3% to 71.3 last month.

Economists polled by Reuters had forecast contracts, which become sales after a month or two, falling 1.0%. Pending home sales declined 2.5% from a year earlier.

"At this critical stage of the housing market, it is all about mortgage rates," said Lawrence Yun, the NAR's chief economist.

The average rate on the popular 30-year fixed-rate mortgage jumped to 6.81% in April from around 6.65% in March, data from mortgage finance agency Freddie Mac showed.