China says it is investigating its largest social media platforms - Weibo, WeChat and Baidu Tieba - for alleged violations of cyber security laws.
The Cyberspace Administration said people had been using the three platforms to spread terror-related material, rumours and obscenities.
The breaches "jeopardised national security," the administration said. China's authorities heavily censor the internet, routinely blocking content or search terms they consider sensitive.
Cyberspace Administration accused internet users of "spreading violence, terror, false rumours, pornography and other hazards to national security, public safety, social order" on the three platforms.
Baidu expressed "regret" and said it would "actively co-operate with government departments" and "increase the intensity of auditing", Reuters reported.
There was no immediate comment from Tencent, which owns WeChat, or Weibo.