Three of China's top Internet portals have pledged to work with the government to banish online rumours, as jittery authorities crack down on the web following widespread rumours of a coup.

Last month, unsubstantiated speculation emerged online about a coup led by security chief Zhou Yongkang, following the dismissal of rising political star Bo Xilai as Communist Party chief of a major city in a rare, public scandal.

Chinese authorities quickly responded by shutting down over a dozen websites, making a string of arrests and punishing the country's two most popular microblogs, run by Sina.com and Tencent.

In comments carried Tuesday on China's state television, Sina, Tencent and Baidu -- another popular web portal -- pledged to "firmly support and cooperate with relevant government departments in cracking down and probing web rumours."

We "must shoulder social responsibility, strengthen supervision of harmful information and adopt effective measures," said Tencent chief administration officer Chen Yidan.

The pledge comes at a time of official nervousness following Bo's sacking, which analysts say has exposed major rifts in China's ruling party ahead of a key, once-in-a-decade leadership transition later this year.

Bo was removed as party chief of the southwestern metropolis of Chongqing last month after his former police chief fled to a US consulate and reportedly demanded political asylum.

The news was only lightly covered by China's tightly controlled state media, opening the way for lurid online speculation involving gunshots and even tanks rolling into central Beijing, and prompting the web crackdown.

On March 31, Tencent stopped users from posting comments to other people's posts for four days -- as did Sina.com -- when the government blamed the web portal for carrying online chatter speculating about the coup.

China operates a huge online censorship system in an effort to block information it deems sensitive, but the huge popularity of microblogs has posed a huge challenge as news is re-posted before censors can delete it.