The heads of China's largest Internet and technology firms have vowed to stop the "spread of harmful information" on the web after attending a three-day government workshop, state media said Sunday.

Nearly 40 companies, including e-commerce giant Alibaba, online portal Sina and search engine Baidu attended the seminar hosted by the State Internet Information Office, an online watchdog, the official Xinhua news agency said.

During the discussion, which ended Saturday, the bosses reached a "common agreement" to "safeguard" the spreading of positive information online and "strengthen self-management and self-discipline", the report said.

They also agreed to "resolutely curb Internet rumours, Internet pornography, Internet fraud and the illegal spread of harmful information" to develop the web into a "positive and beautiful spiritual home".

Industry and information technology minister Miao Wei also ordered Internet companies to increase their investment in "tracking surveillance" -- reflecting growing anxiety among top leaders over the increasing influence of the web.

The pledge by the private and state-owned companies backs efforts by the government to tighten its grip on the fast-growing Internet sector, which has become a platform for citizens to express their opinions and frustrations.

China's Communist leaders last month ordered stricter control of social networking sites such as Chinese versions of Facebook and Twitter and a crackdown on "vulgar" material on the web.

Communist Party chiefs made the call in a list of "cultural development guidelines", which analysts said were designed to strengthen the government's control of the web and make state-run media more competitive.

The growing popularity of privately owned social networking sites in a country with more than 500 million people online has alarmed Beijing, as more and more web users take to the Internet to vent their anger.

China has repeatedly vowed to clamp down on Internet "rumours" -- often used as code for criticism of the government -- and Beijing said last month that police had begun to detain and punish people for spreading rumours online.

Authorities have also been reining in television entertainment, replacing popular shows with so-called "healthy" programming.

For the past decade Beijing has been encouraging state-run media to be more competitive and less reliant on state subsidies, which has led to more critical reporting and racier programming as outlets compete for readers and viewers.

But the trend towards more free-wheeling reporting has undermined official efforts to control public opinion and unnerved authorities who have seen previously obedient media outlets criticise their decisions.