The pace of growth in the U.S. private sector slowed marginally in January, weighed by declines in its new orders and employment components, while the key services sector reading hit a four-month high, 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 - read 56.0 in January, down from 56.1 in December.

A reading above 50 signals expansion in economic activity.

January's "flash" or preliminary services sector PMI, also reported by Markit on Monday, rose to 56.6 from 55.7 last month. It was the strongest reading since last September's 57.7.

"U.S. service providers reported a busy January, providing an important signal that the economy remains in good health at the start of the year," said Markit chief economist Chris Williamson.

"Growth of business activity picked up from the already-robust pace seen in December, and optimism about prospects for the year ahead rose to one of the highest levels we've seen since the financial crisis," he said about the "flash" report.

The dip in the main index readings was due in part to a fall in the composite employment gauge to 53.9 from 55 last month.

The employment component of the services sector slowed to 54.1 from 55.2.

The January non-farm payroll report is scheduled for release on Feb. 7. A Reuters survey of economists shows an expectation for 175,000 jobs created in the month.

Markit's "flash" reading is based on replies from about 85 percent of the U.S. companies in the services sector that were surveyed.