A U.S. non-manufacturing activity indicator hits 21-year high in September, signaling robust growth of the economy in the month, according to data released by the Institute for Supply Management (ISM) on Wednesday.

The Non-Manufacturing Index (NMI), which measures activity in the U.S. service sector, rose 3.1 percentage points to 61.6 in September, reflecting growth for the 104th consecutive month, according to the ISM.

The NMI survey covers all sectors outside of manufacturing. A reading above 50 percent indicates expansion of the service sector.

The business activity index went up 4.5 percentage points to 65.2; the new orders component of the index, a signal of future business, rose 1.2 percentage points to 61.6; and the employment index also increased 5.7 percentage points to 62.4 in September, according to the survey.

Of all the 17 non-manufacturing industries tracked by the NMI, no industry reported a decrease in September.

Respondent comments continue to indicate a positive outlook for business conditions and the economy, but concerns about capacity, logistics and the uncertainty with global trade remain, said the ISM in the report.

The U.S. economy grew at an annual rate of 4.2 percent in the second quarter of the year, despite negative impacts from Hurricane Florence.

The Fed expected the U.S. economy to grow at 3.1 percent this year, higher than 2.8 percent estimated in June, according to the Fed's latest economic projections released last week.