BEIJING, July 6 (Reuters) - China will step up supervision of Chinese firms listed offshore, and improve regulation of cross-border data flow and data security, Xinhua news agency quoted the cabinet as saying on Tuesday. China will also crack down on illegal activities in the securities market and punish fraudulent securities issuance, market manipulation and insider trading, Xinhua said. China will check sources of funding for securities investment and control the leverage ratio, it added. (Reporting by Colin Qian, Judy Hua and Kevin Yao; Editing by Andrew Heavens) Our Standards: The Thomson Reuters Trust Principles.