Three popular Chinese internet services have been ordered to stop streaming video after censors complained it contained improper comments on sensitive issues.

The move prompted a sell-off in the U.S.-traded shares of Sina Corp. and its microblog service, Sina Weibo. Thursday's announcement adds to efforts by President Xi Jinping's government to tighten media control ahead of a Communist Party congress late this year. Xi is due to be appointed to a second five-year term as party leader.

Video streamed by users of Sina Weibo, AcFun and Phoenix New Media's ifeng.com contained "negative comments" about unspecified sensitive issues, the State Administration of Press, Publication, Radio Film and Television said. It ordered them to stop the services. Communist leaders promote internet use for business and education but try to block access to material deemed subversive or obscene. Beijing has been especially wary of social media since they were used by organizers of the Arab Spring protests that spread across the Middle East in 2010 and led to the downfall of the Egyptian and Tunisian governments.

Rules that took effect June 1 bar private or foreign companies from directly disseminating news or investing in online news services. Those that want to work with foreign partners must undergo a security review.