WASHINGTON — Growth at US service companies slowed slightly in January behind weaker new orders and business activity. But hiring improved, a bright sign for the economy.

The Institute for Supply Management said Tuesday that its index of nonmanufacturing activity dipped to 55.2 in January.

That’s down from 55.7 in December, which was the highest level in nearly a year. Any reading above 50 indicates expansion.

The modest decline from December’s strong reading suggests the industry was not greatly hampered by an increase in Social Security taxes that reduced take-home pay for most Americans.

The report measures growth in industries that cover 90 percent of the workforce, including retail, construction, health care, and financial services.

Overall, economists were encouraged by the steady reading in the services index.