Hiring by U.S. small businesses weakened in September for a third straight month, weighed down by economic uncertainty, suggesting lackluster job gains in the overall economy.

The National Federation of Independent Business said the net change in employment per firm slipped 0.23 last month after declining 0.05 in August.

"Uncertainty has cast a cloud over the future for owners, making it difficult to make commitments to new spending and hiring," the NFIB said in a statement.

The survey was released ahead of the government's more comprehensive payroll count on Friday. Nonfarm payrolls likely increased 113,000 in September, according to a Reuters poll, after rising 96,000 in August.

The unemployment rate is seen rising a 10th of a percentage point to at 8.2 percent. The NFIB survey showed a marginal decline in the share of employers reporting difficulties filling job openings last month.