A key gauge of U.S. manufacturing activity likely rose in August, which would bring the index into positive territory for the first time since just after the recession began, according to a Reuters' poll of economists.

The Institute for Supply Management's manufacturing index is expected to have risen to 50.5 in August from 48.9 in July, according to a median forecast from a recent Reuters poll.

Estimates from the 78 economists polled ranged from a low of 48.9 to a high of 53.5.

A reading above 50 indicates the sector is expanding.

An expansionary reading on the nationwide ISM manufacturing index would come a day after the Institute for Supply Management-Chicago reported that its midwest business barometer rose to 50.0 in August.

The national Institute for Supply Management will release its August non-manufacturing index at 10 a.m. (1400 GMT) on Tuesday.

(Polling by Bangalore Polling Unit)

(Reporting by Ellen Freilich and Camille Drummond; Editing by Dan Grebler)