People read on mobile phone while waiting for the train in Tianjin railway station.

Chinese authorities said on Monday that they have handed down maximum fines to the operators of three major social-media platforms in the country for failing to deal with pornography, violence and other banned content on their sites.

The affected platforms are Baidu's online forum Tieba, microblogging site Weibo and Tencent's massively popular social app WeChat. In August, authorities had said the respective operators were under investigation for cybersecurity violations.

The Cyberspace Administration of China, the watchdog that monitors online activities, issued a notice saying tech giants Baidu and Sina Weibo were handed maximum fines under the new cybersecurity law for "failing to fulfill their management duties" in dealing with pornographic and violent content, as well as information that "promotes ethnic hatred."

Separately, the watchdog's Guangdong office issued a similar notice that handed the same maximum fine to Tencent.

The regulators did not specify the maximum amount that the companies could be fined. But under the new cybersecurity guidelines that went into effect earlier this year, it could be up to 500,000 yuan ($76,000) — a relatively small amount for Baidu, Tencent and Weibo. For context, Tencent's total revenue for 2016 was $21.9 billion.

Tencent and Weibo did not immediately return CNBC's requests for comment.

A Baidu spokesperson referred CNBC to a statement the company made in August when regulators began their investigation. In the statement, the company apologized and said it would work with authorities to rectify the situation and improve the platform's information verification efforts.

Separately, Facebook-owned messaging service WhatsApp seemed to be functioning properly Tuesday morning after it earlier appeared to have been blocked again on the mainland. Users had said then that they were unable to access the service without a virtual private network (VPN) connection that usually helps to bypass censorship.

Some reports indicated the service may have been unavailable since September 20. WhatsApp was previously blocked by China in July before it resumed service.

Facebook did not immediately return CNBC's request for comment.

In January, China's Ministry of Industry and Information Technology embarked on a 14-month campaign to "clean up" China's internet connections by March 31, 2018. The ministry had said that, while China's internet access service market is facing "a rare opportunity for development," there are also signs of "disorderly development" needing to be rectified.

New cybersecurity measures went into effect in June in a move touted by Beijing as a milestone in data privacy regulations, but critics said authorities hadn't provided enough information about how the wide-reaching law would be implemented.

In recent months, China has raised the pressure on the country's internet space in what some say is an attempt to exert control in the lead up to the Communist Party Congress next month.

Beijing doubled down on its crackdown of the domestic internet in July, after news had emerged that Apple pulled several VPN services from the local version of the App Store in a move that was met by a wave of online criticism.

Reports also said that authorities went after China's top live-streaming services in July.

— CNBC's Barry Huang contributed to this article.