U.S. private sector economic activity growth slowed slightly in December, with the services sector reading also edging lower, an industry report showed on Monday.

Financial data firm Markit said its composite Purchasing Managers Index (PMI) — a weighted average of its manufacturing and services indexes — measured 56.1 in December, down just slightly from the 56.2 posted last month, as well as the "flash" or preliminary December reading of the same amount.

A reading above 50 signals expansion in economic activity.

Last month's services sector PMI, also reported by Markit on Monday, dipped to 55.7 from 55.9 last month, and was lower than the flash reading of 56.

Despite the slight dip in the main index readings, the pace of growth in hiring advanced for both the services sector and the total private sector. The composite employment gauge rose to 55 from 52.4 last month, while the read on services employment rose to 55.2 from 52.4 in November, which was the lowest since March.

"The U.S. economy appears to have ended 2013 on a strong note, especially in relation to hiring," Chris Williamson, Chief Economist at Markit, said in a press release.

The upturn in the rate of job creation "sets the scene for another upbeat non-farm payroll report for December," he said.

The December non-farm payroll report is scheduled for release on Friday, and a Reuters survey of economists shows an expectation for 197,000 jobs created in the month.