The U.S. services sector grew in January for a third year of back-to-back monthly expansion, although the pace of growth slowed slightly from the previous month, according to a report released on Tuesday.

The Institute for Supply Management said its services sector index eased slightly to 55.2 last month from 55.7 in December. The reading was in line with economists' forecasts, according to a Reuters survey.

A reading above 50 indicates expansion in the sector, while one below 50 indicates contraction. The index was last below 50 in December 2009 as the U.S. economy was beginning to recover from the financial crisis and recession.

However, there were some signs of weakness in the index's underlying components. The new orders index, seen as a forward-looking measure, was its lowest since April 2012, falling to 54.4 in January from 58.3 the month before.

The business activity index also fell, dipping to 56.4 from 60.8 the month before and hitting its lowest since August 2012.

The employment index, however, was at highest since February 2006. The index rose to 57.5 in January from 55.3 in December.