Pending sales of existing U.S. homes in May unexpectedly fell to their lowest level in more than 5-1/2 years, data from a real estate trade group showed on Tuesday in a sign of continued weakness in the housing sector.

The National Association of Realtors said its Pending Home Sales Index, based on contracts signed in May, fell 3.5 percent to 97.7 from a downwardly revised level of 101.2 in April. The May index is the lowest since 89.8 in September 2001.

Wall Street analysts polled ahead of the report were expecting the May index to rise 0.2 percent.