U.S. small business optimism slipped in September as business owners worried about the economy's near-term outlook, but remained fairly upbeat on sales and expansion prospects.

The National Federation of Independent Business said on Tuesday its Small Business Optimism Index fell 0.2 point to 93.9 last month.

The NFIB corrected August's reading to show 94.1 rather than 94.0, saying March data was mistakenly used in August's calculations.

The September survey was conducted through September 30, when funding for the government ran out, leading to a partial shutdown of federal government activities on October 1.

"It is possible that the pending government shutdown weighed on the economic outlook," the NFIB said in a statement. "Now it is a reality, so the only issue is its duration and where the politicians decide to cut back to hurt their opponents."

While the index dipped in September, five of its components - including sales and capital spending - increased.

There were also gains in the number of owners who believed this was a good time to expand and those saying they could not find suitable candidates to fill open positions.

The improvements should offer hope that the economy continues to grow steadily after output expanded at a 2.5 percent annual rate in the second quarter.

While owners were optimistic about sales, they were downbeat on earnings. Expectations for business conditions over the next six months fell again in September.