An index tracking broad U.S. economic activity improved in December but still suggested the economy remained in recession, the Federal Reserve Bank of Chicago said Monday.

For the month of December, the National Activity Index rose to -0.77 from a revised -1.29 in November, the highest reading since -0.22 in July 2000. But the index was weaker than the -0.53 reading in December 2000.

The three-month moving average of the Chicago Fed index rose to -1.05 in December from a revised -1.44 in November, but was sharply weaker than the reading of -0.59 a year ago.

The latest reading on the three-month average, considered the more reliable of the gauges since it smooths out monthly fluctuations, was the highest since -0.95 last August.

"The index continues to be in a negative range that historically has been associated only with economic recessions," the Chicago Fed said in its report.

December marked the 18th straight month that the three-month average was below zero and that the economy has been growing below trend, it said.

The Chicago Fed index is based on 85 economic indicators that cover areas including production, income, employment, consumption, housing and manufacturing.

Sixty-four of the 85 indicators showed below-average growth in December, the report said.