China’s internet authority opened an investigation into Tencent, Baidu and Sina Weibo amid allegations the companies’ social media services were being used to spread illegal content.

In a statement, the Cyberspace Administration of China said the companies were under investigation for breaking the country’s Internet Security Law and other regulations around the distribution of content deemed inappropriate.

According to the authority, the platforms are suspected of facilitating the spread of content related to: “violent terror, false rumours, pornography and other hazards to national security, public safety, social order.”

The investigation relates to user-generated content on Tencent’s WeChat platform, Sina Weibo’s microblogging site and social media platform Baidu Post – three of the largest social networking websites operating in the country. The watchdog said it had written to the companies about the accusations.

In an apparent warning to companies in the sector, it added the government would: “Conscientiously implement the Internet Security Law and other laws and regulations, and further increase the internet information content supervision and law enforcement efforts to investigate and deal with all kinds of illegal acts online.”

China’s Internet Security Law was introduced in June and requires – among other rules – service providers to monitor content posted on their platforms and adopt systems allowing them to trace the source of content.

Companies breaking regulations face a large fine or, in extreme cases, the suspension of business activities.