WASHINGTON (AP) — Service companies continued to expand at a healthy pace last month and stepped up hiring, providing more evidence that the American economy was growing and adding jobs.

The Institute for Supply Management said Wednesday that its index of nonmanufacturing activity dropped to 56 in March, from its 12-month high of 57.3 in February. Any reading above 50 indicates expansion.

Economists said the slight decline was not troubling because the reading was still at a healthy level that points to growth. And a measure of employment rose to near a six-year high reached in January.

I.S.M., a trade group of purchasing managers, surveys roughly 90 percent of American companies in all sectors outside of manufacturing. That includes retail, construction, financial services, health care and hotels.