BEIJING, CHINA - China's censors on Tuesday ordered the nation's most popular Internet search engine to apologise for allowing explicit photos of Hong Kong celebrities to be published on its site.

NASDAQ-listed Baidu came under heavy criticism for "spreading" the photos of several leading Canto-pop and film stars in compromising positions, which have been plastered across the Internet.

The Beijing Association of Online Media said in the statement that "we severely criticise Baidu's behavior," state news agency Xinhua reported.

"'Key-words searching' and 'Tieba', a picture-sharing section of Baidu.com, became a platform to show and spread the obscene pictures," the statement said.

"And Baidu failed to block the photos after other Beijing-based websites had taken actions against the pictures' spreading," it added.

At least seven stars have been sucked into the twists and turns of the sex scandal over the photos, which feature singer-actor Edison Chen with a string of Hong Kong starlets including pop singer Gillian Chung, actress Cecilia Cheung, and former actress Bobo Chan.

Chen has been romantically linked to all three in the past. Media reports say the photos were copied from Chen's computer when he sent it in for repairs.

Last week, Chen issued a statement on video website YouTube, in which he apologised to "anyone who has been affected by this strange, strange ordeal."

The police have made several arrests linked to the case, but have faced criticism that they have been over-zealous. A charge against one Hong Kong man detained over the photos was dropped last week.

Baidu are currently in a legal battle with some of the world's biggest music labels, who accuse the website of providing links to pirated music files.

Media in China are tightly controlled, but censors have often struggled to eliminate unfavourable or controversial content on the Internet.