Factory activity in the U.S. Mid-Atlantic region plummeted in August, falling to the lowest level since March 2009, a survey showed on Thursday.

The Philadelphia Federal Reserve Bank said its business activity index dropped to minus 30.7 from positive 3.2 the month before and was far below economists' expectations for positive 3.7, 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 larger national report by the Institute for Supply Management.