China has vowed to step up its ongoing crackdown on mobile WAP sites offering pornography for mobile phone users, as such sites are taking measures to avoid supervision.

An official from the national office against pornographic and illegal publication told Xinhua Thursday that the office had ordered local authorities to "fully clean up" such WAP sites and shut down those with "serious violations". Cases suspected of constituting a crime shall be handed over to police.

The most difficult part in the crackdown is to discover mobile WAP sites spreading pornography, the official said.

"Most Internet supervisory bodies do not have equipment to watch over mobile WAP sites, and manual check using mobile phones is rather slow," the official said.

The official added a large number of mobile porn sites were moving their servers to overseas to avoid supervision.

The crackdown started on Nov 16 and will last until the end of this year, with focuses on Shanghai, Beijing, Guangdong, Zhejiang and Anhui, where many WAP sites are registered.

China Mobile, the country's largest wireless carrier, has suspended cooperation with 460 "business promotion partners" since the crackdown was launched.