US manufacturing activity slowed for the third straight month in September to a near-stall, with companies expressing concern about China's slowdown and the global economy, a private firm reported Thursday.

The Institute for Supply Management said its purchasing managers index for the manufacturing sector fell to 50.2 in September, the lowest level of this year to date, from 51.1 in August.

A PMI reading below 50 indicates contraction.

Of the 18 manufacturing industries surveyed last month, only seven reported growth, led by printing and textile mills, while the other 11 reported contraction.

The sharpest drops were in the backlog of orders, down 5.0 percent, followed by production and new orders. Exports activity continued to contract at the same pace, while imports fell more than in August.

Survey respondents expressed worries about the slowing Chinese economy and its effect on US consumer confidence, as well as the impact from low crude oil and natural gas prices.

"Overall, business is slowing. Consumers are nervous. Not sure what is coming next," said one respondent in the transportation equipment industry.

Barclays analyst Jesse Hurwitz noted the PMI was now down to a two-year low.

"We view the September ISM data as consistent with the regional surveys already reported that show a stagnant US manufacturing sector," he said.

"Lower energy prices, a strong dollar, and weak international demand are likely to prevent a strong rebound for the sector in the near term."