China's cyber regulator on Friday said it was investigating the country's top social media sites over failing to comply with strict laws that ban content which is violent, obscene or deemed offensive to the Communist Party.

The Cyberspace Administration said it was investigating Tencent Holdings Group Ltd's WeChat, Weibo Corp and Baidu Inc's forum site Tieba over suspected violations of the country's strict cybersecurity laws.

“Users are spreading violence, terror, false rumours, pornography and other hazards to national security, public safety, social order,” the regulator said on its website.

The companies did not immediately respond to emails and phone calls seeking comment on the probe.

This is the latest in a series of regulatory actions against the country's top tech firms as China's cyber authorities adopt an increasingly hardened stance on censorship, doling out harsh punishments to firms that fail to comply.

Cyber surveillance is being tightened further ahead of the 19th National Congress of the Communist Party of China expected to be held later this year, when global attention will be on news coming from the world's second-biggest economy.

Last month, cyber authorities called on the same firms to carry out immediate “cleaning and rectification” at a meeting with their representatives, where the authorities cited specific examples of illicit content, including rumours about party officials and misrepresenting Chinese military history.

Prior to the meeting, Weibo was ordered to partially close its video site over violations, wiping out a combined $1.3 billion worth of stock between Weibo and parent firm Sina Corp .

Messaging app WeChat and microblogging service Weibo are China's most popular social media platforms, and have thrived due to the absence of western competitors like Facebook and Twitter that are banned by the country's censors.

WeChat and Weibo have about 940 million and 350 million monthly active users, respectively.