"The September PMI reading in fact rounds off the strongest quarter recorded by the survey since the financial crisis, narrowly beating the previous peak seen when the economy was rebounding from recession in the second quarter of 2010," Markit chief economist Chris Williamson said.
The output subindex also turned down to 59.6 in September from 60.7 during August, while the employment component rose to 56.4 from August's final read of 54.6.
"The labor market upswing clearly bodes well for overall non-farm payroll growth to rise above 200,000 in September," Williamson said.
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"Recent months have also seen a welcome revival in export growth, suggesting trade should also help boost the U.S. economy in the third quarter," he said. "GDP looks set to grow by at least 3.0 percent in the third quarter, with good momentum being sustained as we move into the final quarter of the year."
A read on new orders turned down modestly last month from August, Markit said.