Private-sector payrolls increased by a less-than-expected 133,000 workers in May, according to the ADP employment report released Thursday. Economists polled by Reuters had forecast a gain of 148,000.

The April level was revised lower to a gain of 113,000 from a prior estimate of 119,000. Markets look to ADP's report to provide some guidance on the U.S. Labor Department's jobs estimate, which will be released Friday.

The modest rise in private employment suggests that the national unemployment rate probably did not decline in May unless the labor force continued to decline, Joel Prakken, chairman of Macroeconomic Advisers LLC, said in a statement.