Business activity in the U.S. Midwest expanded less robustly in December than originally thought, an industry association reported on Monday, with employment and new orders registering less growth than earlier reported.

The Institute for Supply Management-Chicago said December's revised business barometer index reading registered 66.8, less than the reading of 68.6 it originally reported last month, which had been the strongest level since July 1988.

Detailing revisions based on seasonal factors, the ISM-Chicago said its employment index was 58.4 in December, below the 60.2 reported earlier. New orders registered a revised 68.7, compared with the 73.6 reported earlier.

Any reading above 50 indicates expansion.