China's government has threatened to shut down Sina, one of the country's most popular news websites unless it "improves censorship", state media reported, in a rare public glimpse into controls over the press.

The online portal "distorted news facts, violated morality and engaged in media hype", the official Xinhua news agency on Saturday cited the Cyberspace Administration of China (CAC) as saying.

The CAC will "seriously" punish Sina, with possible measures including "a complete shut down of its Internet news services", Xinhua added.

CAC officials added that "censorship of user accounts has been poor", Xinhua said, in a likely reference to Sina Weibo, a service similar to Twitter which has hundreds of millions of registered users in China.

The report did not provide specifics on which of Sina's news offerings had fallen foul of censors, but said the CAC accused Sina of spreading "illegal information related to rumors, violence and terrorism", and "advocation of heresies".

Chinese authorities have in the past used "heresy" to refer to content related to banned religious groups, such as the Falun Gong.

The Chinese government generally operates its control over media behind the scenes, with secret directives on how to report stories. Journalists who disobey or leak the orders can be punished.

Controls have tightened under China's current president Xi Jinping. The France-based group Reporters Without Borders ranked China 175 out of 180 countries in its 2014 worldwide index of press freedom.

"Chinese web giant Sina will face suspension of its Internet news services if it fails to improve censorship," Communist party mouthpiece the People's Daily wrote on Twitter, a site which is blocked by Chinese authorities.

China in 2013 launched a crackdown on "online rumours", with several people posting content deemed untrue jailed in a campaign seen as an attempt to rein in online debate on microblogging services.

The campaign prompted a number of prominent government critics to quit microblogging or tone down their comments, and was blamed for a drop in Sina Weibo use.

Sina's portal is the fourth most visited website in China, according to ranking service Alexa. Neither Sina nor CAC could not immediately be reached for comment on Sunday.