Beijing ? China warned Google and other popular Web portals Monday that they must do more to block pornographic material from reaching Chinese users, the latest in a series of government crackdowns targeting Internet content.

The crackdown focused on pornography but is part of a larger Chinese effort to control freedom of expression and root out material it considers destabilizing, such as sites that criticize the Communist Party, promote democratic reform or advocate Taiwan independence.

Pornography is banned in China but remains widely available on and off the Internet. Popular Chinese Web portals frequently show sexually explicit pictures and provide links to pornographic Web sites.

The announcement said Google and Baidu, China’s two most heavily used search engines, had failed to take “efficient” measures after receiving notices from the country’s Internet watchdog that they were providing links to pornographic material.

Google asserted that it abides by Chinese law and does not generate pornographic content.