Small businesses cut back on staff slightly in May, the first decline in six months and a sign of weakness in the job market recovery.

The National Federation of Independent Business said on Wednesday employment shrank by 0.04 workers per firm last month.

It was the second report of the day to augur poorly for the government's national employment report due on Friday, which is expected to show a modest 170,000 jobs were created last month.

Earlier on Wednesday, payroll processing firm ADP said U.S. companies added 135,000 workers in May, well below the 165,000 forecast by analysts polled by Reuters.