SHANGHAI, China — A group of 14 media companies calling themselves the Beijing Association of Online Media has voluntarily agreed to self-censor any objectionable content that appears on its sites. The pledge comes amid calls in Beijing to clean up the Internet.

Sina Corp. and Tom Online, Sohu.com Inc., NetEase.com Inc., Baidu.com Inc. and Yahoo Inc.s China Web portal have all decided to stringently police sites for unhealthy content according to the Communist Party.

“At the same time as the web develops quickly, certain sites are transmitting unhealthy news ... and uncivilized voice services, including pornographic content that can be harmful to society,” said the pledge posted on Sina’s site.

A spokesman for Tom Online said the new pledge could affect 40 percent of its online offerings, but the financial impact will be limited.

“The Internet is a very small part of the business, about 5 percent,” he told CNN.com. Tom makes most of its money from mobile content offerings.

China, with a population more than 1 billion, has 100 million Internet users. The Chinese government heavily regulates Internet access. Websites that publish pornographic or subversive material are strictly forbidden.

In late March a Chinese government task force announced it shut down or fined dozens of sites that carried pornographic content or images under copyright. According to authorities, 53 sites were closed, with no further details given.

Also in March, a coalition led by China Netcom banded together with 11 other networks to create a unified filtering system. China is no longer a member of ICANN.