(RTTNews) - Activity in the U.S. service sector unexpectedly grew at a slightly faster pace in the month of September, according to a report released by the Institute for Supply Management on Tuesday. On the inflation front, the prices index advanced to 77.5 in September from 75.4 in August, indicating an acceleration in the pace of growth. The ISM released a separate report last Friday showing an unexpected acceleration in the pace of growth in U.S. manufacturing activity in the month of September.