China's cyber watchdog has handed down maximum penalties to several of the country's top tech firms, including Tencent Holdings Ltd, Baidu Inc and Weibo Corp, for failing to properly censor online content.

The rebuke comes as China is stepping up censorship and security efforts ahead of the 19th National Congress of the Communist Party, a major leadership conference held once every five years.

Notices posted by the Cyberspace Administration of China on Monday said the firms would receive the "maximum penalty" for failing to remove fake news and pornography as well as content that "incites ethic tension" and "threatens social order."

It is the first time that the CAC has levied the maximum fines against tech firms under a new law introduced in June, as it seeks to tighten its grip on the internet.

"The internet does not operate outside of the law... the CAC will seriously implement the new cybersecurity law and other regulations to increase territorial supervision and enforcement efforts regarding the internet," said the CAC.

The notices did not detail the exact sum of the penalties, but under the rules cited in the notice individuals in charge of the platforms and others directly involved face a maximum penalty of 100,000 yuan ($15,110) each.

The law also stipulates that offending platforms can have their licenses revoked and their services suspended for failing to comply.

The measures are the result of an investigation launched last month by the CAC, and target the country's most popular social media services, including Tencent's WeChat and Weibo's microblogging service, whose combined registered users exceed 1 billion.

Earlier this month the cyber watchdog also introduced new regulations that require messaging apps to create credit scores that can be lowered to punish and restrict users who post offensive content. The rules, which take effect on October 8, also make users legally liable for content deemed illegal.

In a recent notice the CAC said illegal content includes articles that "misinterpret government policy," "twist the history of the Chinese Communist Party" and "flaunt excessive wealth."

According to Monday's notices the firms will be required to undertake immediate rectifications to remove illegal content and accounts as well as increase content auditing measures.