Factory activity in the U.S. mid-Atlantic region unexpectedly contracted in January, a survey showed on Thursday.

The Philadelphia Federal Reserve Bank said its business activity index fell to -5.8 from 4.6 the month before. That was well below economists' expectations for a rise to 5.8, according to a Reuters poll.

Any reading above zero indicates expansion in the region's manufacturing. The survey covers factories in eastern Pennsylvania, southern New Jersey and Delaware.

It is seen as one of the first monthly indicators of the health of U.S. manufacturing leading up to the national report by the Institute for Supply Management.