Two Chinese political websites said Friday they had been ordered by authorities to shut for a month for criticising state leaders, the latest move in a broad government crackdown on the Internet.

Officials told the Mao Flag website, named after late leader Mao Zedong, and the Utopia website, also known for a leftist political stance, to close for "rectification", the sites said in separate announcements.

Authorities said their postings had "maliciously attacked state leaders" and given "absurd views" about politics, according to statements posted on the websites.

Those statements, dated Friday, were later removed. The operators could not be reached for comment and content on the sites was unavailable.

The latest crackdown comes after a surge in online rumours in China, including one about a coup led by security chief Zhou Yongkang, following the March dismissal of rising political star Bo Xilai.

Two other sites, China Elections and April Youth, also appeared to be shut on Friday but operators claimed they were down for maintenance and staffing reasons.

The Utopia website was a supporter of the policies of Bo, who was removed as Communist Party chief of the southwestern metropolis of Chongqing in mid-March.

During his time in the city, he ran a hardline crackdown on crime and a populist Maoist revival campaign that included singing patriotic songs, which was praised by Utopia.

China launched a sweeping Internet crackdown last week, highlighting official unease ahead of a leadership transition later this year.

Efforts to quell "rumour-mongering" come as President Hu Jintao and China's other top leaders step down from their Communist Party posts in a secretive 10-yearly leadership transition that will culminate in early 2013.

Authorities shutdown 16 other websites, arrested six people and slapped temporary curbs on two popular microblog services, preventing users from posting comments.

China, which has the world's largest online population with over half a billion users, has long blocked content it deems politically sensitive as part of a vast censorship system known as the Great Firewall.

But the rise of social media, in particular Twitter-like microblogs, have proved more difficult to control and have become a popular outlet for expressing discontent towards the government.