WASHINGTON, March 31 (Reuters) - Contracts to buy U.S. previously owned homes fell for a second straight month in February, signaling a cooling in the housing market in the months ahead as accelerating prices amid tight supply and rising mortgage rates reduce affordability. The National Association of Realtors (NAR) said on Wednesday its Pending Home Sales Index, based on contracts signed last month, tumbled 10.6% to 110.3, with contracts falling in all four regions. Economists polled by Reuters had forecast pending home contracts, which become sales after a month or two, would decline 2.6% in February. Compared to a year ago, pending home sales slipped 0.5% in February. Contracts had increased for eight straight months on a year-on-year basis. The decline in contracts suggested sales of previously owned homes could fall further in March after dropping sharply in February. The supply of existing homes is at a record low. The 30-year fixed-rate mortgage has risen to a nine-month high of 3.17%, according to data from mortgage finance agency Freddie Mac. Though mortgage rates remain historically low, the sustained increase since February is contributing to making home ownership more expensive for first-time buyers. Mortgage rates have risen in tandem with U.S. Treasury yields, which have spiked in anticipation of stronger economic growth this year and higher inflation. Data on Tuesday showed the S&P CoreLogic Case-Shiller house price index soared 11.2% in January from a year ago, the fastest in 15 years, after rising 10.4% in December. Demand for housing is being driven by Americans seeking more space for home offices and schooling as the year-long COVID-19 pandemic drags on. But builders are constrained by record-high lumber prices as well as shortages of land and labor. Last month, contracts declined 9.2% in the Northeast and tumbled 9.5% in the Midwest. They plunged 13.0% in the densely populated South, which was slammed by snowstorms in February. Pending home sales fell 7.4% in West. (Reporting by Lucia Mutikani Editing by Paul Simao)