Contracts to buy previously owned U.S. homes slipped in August due to a shortage of lower priced inventory in most of the country, an industry group said on Thursday.

The National Association of Realtors said its Pending Home Sales Index, based on contracts signed in August, fell 2.6 percent to 99.2, but was 10.7 percent higher than last year.

July's reading was revised up to 101.9, the highest level since April 2010, when buyers were racing to use the home-buyer tax credit before the deadline, the group said.

"The performance in month-to-month contract signings has been uneven with ongoing shortages of lower priced inventory in much of the country," the association's chief economist, Lawrence Yun, said in a statement.