The Institute for Supply Management says its index of non-manufacturing activity rose to 52.6 in December, up from 52 in November. Any reading above 50 indicates expansion.
The trade group of purchasing managers surveys a range of businesses, including restaurants, hotels, retailers, financial service firms and construction companies.
The service sector has grown for two straight years. But activity had slowed for a third straight month in November, dropping to the lowest reading since January 2010.