China has launched probes into three of its largest social networking platforms over the suspected dissemination of violence and obscenity — the latest move aimed at sanitising the country's increasingly closed-off internet.

The world's most popular messaging service WeChat, the Twitter-like Weibo as well as the Tieba discussion forum are being investigated, according to an announcement from the Cyberspace Administration of China on Friday.

Citing reports from internet users, the administration said other users on WeChat, Weibo and Tieba's platforms "have disseminated content showing violence, terrorism, fake rumours, obscene pornography and more".

Such materials "endanger national security, public security and the social order" and are illegal under a cybersecurity law that came into force in June, the agency said.

The companies acknowledged the investigation in statements Friday, and all three apologised to their users for any "negative experiences" caused by material that "corrupted" the social networks.