WASHINGTON DC, United States — The US service sector expanded in March for the 16th straight month, although growth slowed from the previous month’s pace, which was the fastest in more than five years.

The Institute for Supply Management, a private trade group, said yesterday that its index of service-sector activity dropped to 57.3 last month, from 59.7 in February. That’s the first decline in seven months. Still, any reading above 50 indicates expansion.

The sector employs about 90 per cent of the work force, with a range of industries that include retail, health care, financial services and construction. The index plummeted to 37.6 in November 2008, at the height of the financial crisis.

The trade group last week said its manufacturing index expanded for the 20th straight month. Like the service sector index, the March reading of manufacturing activity was slightly lower than the nearly seven-year high hit in February.

Together, the two indices point to an economy that is growing at a steady and modest pace, despite some challenges.

“The report shows a few scattered signs of slowing momentum, but not enough to disrupt the overall picture of continued growth,” said Ryan Wang, an economist at HSBC Securities, about the service index.

One cause for concern was a sharp drop in a measure of current business activity, which dragged down the overall index. Some economists attributed part of that fall to interruptions in shipping and other activity stemming from the March 11 earthquake in Japan. The transportation and warehousing sector was one of two industries, out of 17, to report less business activity in March.

Toyota Motor Corp said earlier yesterday that the lack of parts from Japan will require it to temporarily shut down all its North American factories later this month. Auto dealers are already preparing for a shortage of Toyota models this spring and summer.

Several companies that responded to the survey also expressed concerns about rising fuel prices and the impact of Japan’s earthquake on the supply of computer equipment and other goods.

Still, a measure of prices paid actually dipped in March.

“Overall, most respondents remain confident about the direction of the economy,” said Anthony Nieves, who oversees the ISM service sector index, which is compiled from a survey of purchasing and supply managers at about 375 companies.

A measure of new orders dipped slightly and an employment index also dropped, though the survey found employment at services firms is still growing.

Job growth in the service sector is key to bringing down the unemployment rate. Private services companies — which range from retailers and hotels to law and accounting firms –, added 199,000 jobs in March, the Labor Department reported last week. That was the biggest gain since November 2006, before the recession began.

Overall, the economy added a net total of 216,000 jobs in March, which brought down the unemployment rate to 8.8 per cent from 8.9 per cent in February.