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 Web sites 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 will be severely punished, but did not give details or say how long the campaign will last.

A Google spokeswoman in China, Cui Jin, defended the site's operations, saying it is a search engine and does not generate any pornographic content. The company obeys 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 e-mail 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 Web sites it considers subversive or too political, including The New York Times' Web site on Dec. 19. It was unblocked a couple days later and remained open Monday.

Beijing loosened some media and Internet controls during the 2008 Summer Olympics—gestures that were meant to show the international community that the games had brought greater freedom to the Chinese people. During the August games, China allowed access to long-barred Web sites such as those of the British Broadcasting Corp. and Human Rights Watch. Those Web sites remained open Monday.

In the past the Foreign Ministry has defended China's right to censor Web sites that have material deemed illegal by the government, saying that other countries regulate Internet usage, too.