China's top market regulator said an investigation is underway over Trip.com Group's alleged antitrust conduct.

The State Administration for Market Regulation said Wednesday that it has started a probe into Trip.com after initial investigations.

The regulator said Trip.com, one of China's biggest online travel agencies, allegedly abused its dominant market position and engaged in monopolistic practices. It didn't provide further details on the investigation.

Trip.com tumbled 9% in premarket trading.

The travel giant, which is listed in both Hong Kong and the U.S., has previously been the target of several antitrust probes by regional Chinese authorities. The company was summoned by the market supervision regulators of Guizhou province and Zhengzhou city last year over alleged monopolistic practices.