A private survey says U.S. services firms grew last month at the weakest pace since May.

The Institute for Supply Management reported Thursday that its services index slid to 55.9 last month from 59.1 in October. The November reading was the lowest since May's 55.7. Still, any reading above 50 signals that services firms are expanding.

The survey registered slower growth last month for business activity, new orders and employment in the services sector.

Services have been a source of strength in the economy this year. Services firms have added 2.5 million jobs since October 2014, 89 percent of jobs created over that period, according to the Labor Department.