Hiring in the US private sector decelerated unexpectedly in July, said payroll firm ADP on Wednesday, while pay gains slowed further.

The private sector added 122,000 jobs in July, down from June's revised 155,000 figure and marking a weaker performance than anticipated.

The report comes ahead of a closely watched central bank decision on interest rates later Wednesday, with the Federal Reserve widely expected to start lowering rates in the coming months.

A cooling jobs market would bolster the Fed's case for beginning rate cuts sooner rather than later.

Policymakers want to ensure inflation is coming down sustainably towards their two percent target, while walking a fine line between easing demand and triggering a downturn.

"With wage growth abating, the labor market is playing along with the Federal Reserve's effort to slow inflation," said ADP chief economist Nela Richardson in a statement.

Year-on-year pay increases slowed to 4.8 percent in July, the slowest rate in three years, said ADP.

For those who changed jobs, their pay gains eased to 7.2 percent in July, down from 7.7 percent.

Service-providing industries accounted for most of July's gains, with trade, transportation and utilities adding 61,000 jobs.

But this was partially offset by losses elsewhere, such as in professional and business services.

The information and manufacturing sectors also showed weakness, even as hiring remained solid in construction.