Seven government agencies will work together on the campaign to "purify the Internet's cultural environment and protect the healthy development of minors," said a statement by the information office of the State Council, China's Cabinet.

Pornography is banned in China, though the government's Internet police struggle to block websites based abroad.

The government 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.

The statement also named popular Web portals Sina and Sohu, as well as a number of video-sharing sites and online bulletin boards, that it said contain problematic photos, blogs and postings.

The statement, which was posted to a news and information Web site managed by the State Council, said violators would be severely punished, but did not give details or say how long the campaign would last.

A Google spokeswoman in China, Cui Jin, defended the site's operations, saying it was a search engine and did not generate any pornographic content. The company obeyed Chinese law, she said.

"If we find any violation, we will take action. So far, I haven't seen any examples of violations," Cui said.

Baidu did not immediately return an email seeking comment, and phones at Sina and Sohu rang unanswered.

China has the world's largest population of Internet users with more than 250 million.

The central government has blocked access to many websites it considers subversive or too political, including The New York Times' website on December 19. It was unblocked a couple days later and remained open Monday.