WASHINGTON, DC - Growth in America's service sector slowed in May, the latest sign that the nation's economic expansion may be cooling.

The Institute for Supply Management's services index fell from 63 in April to 60 in May. However, any figure above 50 represents growth in the sector, which accounts for around 80% of all US economic activity.

Manufacturing output levels also dipped in May, although US retailers saw May sales figures beat forecasts.