BEIJING?—?China's service sector growth cooled slightly in June, but remained in expansionary territory, a private survey showed Wednesday.

The Caixin General Services Purchasing Managers' Index (PMI) fell to 51.6 from May's four-month high of 52.8, according to a survey conducted by financial information service provider Markit and sponsored by Caixin Media Co Ltd.

A reading above 50 indicates expansion, while a reading below 50 represents contraction.

This came after an official survey showed continued improvement in the sector, as the non-manufacturing PMI climbed to a three-month high of 54.9 in June.

The official survey samples 4,000 relatively large non-manufacturing companies, while the Caixin survey has a smaller sample size of over 400 companies and mainly focuses on small and medium-sized firms.

Caixin said the slower growth in service-sector business activity reflected a weaker increase in new order volumes.

Despite the slowdown in growth momentum, Caixin said services companies' confidence towards the year ahead improved slightly in June. The 12-month business outlook edged up to a five-month high in June, with some firms linking positive expectations to new projects and forecasts of improving new order intakes.

The service sector - which includes finance, real estate services and marketing, transport and retail - has become an increasingly important part of the Chinese economy, as the country tries to shift the economy towards a growth model that draws strength from consumption, services, and innovation.

The sector accounted for more than half of the Chinese economy last year.