China's top social media sites, including Baidu, Tencent and Weibo, are under government investigation for failing to comply with laws which ban violent and obscene content.
The Cyberspace Administration of China (CAC) said on Friday it was investigating Tencent's WeChat, Sina's Weibo and Baidu's Reddit-like forum Tieba over suspected violations of cyber-security laws.
According to the administration's website, "users are spreading violence, terror, false rumors, pornography and other hazards to national security, public safety and social order".
Cyber authorities also called on the same firms to carry out immediate "cleaning and rectification" at a meeting with their representatives last month.
As one of China's most popular social media platforms, Weibo, was ordered to partially close its video site over violations, wiping out a combined 1.3 billion US dollars worth of stock between Weibo and parent firm Sina Corp.
Compared to the rest of the world, China has its own censorship in terms of accessing information online.
The CAC was responsible for issuing a "voluntary pledge" that was intended to be adhered to by the major Internet portals in China about the comments that would or would not be allowed to be made on their website.
According to CAC policy, information includes "harmed national security... harmed the nation's honor or interest...damaged the nation's religious policies...spread rumors...disturbed public order... and intentionally using character combinations to avoid censorship" are banned.
WeChat and Weibo have about 940 million and 350 million monthly active users respectively, according to official data.
The companies have not issued an official response yet.