Hiring in August was the slowest it's been since December of 2010, which Dobson said partly reflected weak overall trends in output and new orders. The index's employment component fell to 52.4 from 52.7 in July.

 The pace of growth in manufacturing remained sluggish in August as exports declined for a third straight month and firms were slow to add new workers, a survey showed on Tuesday.

The final Markit U.S. Manufacturing Purchasing Managers Index stood at 51.5 this month, below a preliminary estimate of 51.9. A reading above 50 indicates expansion.

A slight increase in output and overall new orders helped nudge the index above 51.4, where it stood at the end of July.

But the pace of growth was still one of the weakest since the sector stopped shrinking in October of 2009. New export orders were a drag on activity, as slow or negative growth in Europe and elsewhere sapped foreign demand for U.S. products.

Mark Wingham, Markit economist, said expansion in the sector was "only modest" in August. Without a significant jump in activity next month, "third quarter manufacturing growth will likely be one of the weakest since recovery began."

The U.S. government will report employment data for August later this week, and the median forecast of economists polled by Reuters is for a gain of 120,000 jobs, down from 163,000 in July.

Some analysts suspect a number below 100,000 could provoke the Federal Reserve to try to boost overall growth with another round of monetary stimulus when it meets in mid-September.

The U.S. economy grew at a 1.7 percent pace in the second quarter, slightly above an initial estimate of 1.5 percent.