Sales of existing homes fell for a third straight month in June, as the number of available homes sitting unsold continued to mount.

Statistics released today by the National Association of Realtors said that sales in June fell to a seasonally adjusted annual rate of 6.6 million homes, down from 6.7 million in May. The number of homes on the market, meanwhile, grew to a 6.8-month supply, up from a 6.4-month supply in May.

The report was seen by many economists as another sign that the overall housing market is slowing down significantly from the torrid pace of 2005.

“Last year’s level of demand and production was unsustainable,” said Michael Carliner, an economist with the National Association of Home Builders. “What we’re hearing from builders across the country, but not everywhere, is that things are slowing down.”