A three-month moving average of a gauge of U.S. economic activity, though below its historical trend, has neared levels seen at the end of previous recessions, a Chicago Federal Reserve report said on Monday.

However, on a monthly basis, economic activity slipped in September according to the Chicago Fed's National Activity Index.

The index weakened to minus 0.81 in September from a revised minus 0.65 in August, which was previously reported at minus 0.90.

The three-month moving average, which smooths out monthly volatility, firmed to minus 0.63 in September from August's revised figure of minus 0.96, previously reported at minus 1.09. The Chicago Fed said that in the past four recessions, the three-month average's rise back above minus 0.70 has coincided closely with the end of the recession.

Zero values in the index indicate a national economy expanding at historical trends, while negative values indicate below-trend growth and positive values signal growth above trend, the Chicago Fed said.

Thirty two of the 85 individual indicators in the index made positive contributions to the index in September.

Manufacturing capacity utilization increased to 67.5 percent in September from 66.8 percent in August, according to the report.

Employment-related indicators made a contribution of minus 0.53 to the index in September versus minus 0.61 in August.

The consumption and housing category's contribution to the index deteriorated in September to minus 0.52 from minus 0.38 in August.

The amount of economic slack in the data indicates low inflationary pressure from economic activity over the coming year, the Chicago Fed said.