The internet giant Yahoo has been accused of providing China with information that led to the jailing of a second internet writer.
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Media watchdog Reporters Without Borders claims that Yahoo released data which led to the arrest of Li Zhi.
The online writer was jailed for eight years in 2003, after posting comments that criticised official corruption.
Last year Yahoo was accused of giving information to Beijing which led to the imprisonment of reporter Shi Tao.
Reporters Without Borders called on Yahoo to release the names of all internet writers whose identities it has revealed to the Chinese authorities.
Yahoo spokeswoman Mary Osako insisted that in its dealings with China, the company "only responded with what we were legally compelled to provide, and nothing more".
"We were rigorous in our procedures and made sure that only the required material was provided," she told the AFP news agency.
But she added that: "The government of China is not required to inform service providers why they are seeking certain information, and typically does not do so."
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Reporters Without Borders said it was not acceptable for the firm to say it simply responded to requests from the authorities without knowing what the data would be used for.
"This argument no longer holds water," the group said in a statement. "Yahoo certainly knew it was helping to arrest political dissidents and journalists, not just ordinary criminals."
The Chinese government enforces strict laws on internet use, blocking content it considers a threat, including references to the Tiananmen Square massacre and notable dissidents.
But major international firms wanting to do business in China, the world's number two internet market, are coming under increasing pressure from rights groups not to conform to Beijing's conditions.
Four major US-based companies - Microsoft, Google, Yahoo and Cisco - have been accused of collaborating with China to censor the internet.
In September, Yahoo was accused of helping the Chinese authorities identify Shi Tao, who was sentenced to 10 years in prison in April 2005.
Google came under fire last month after it announced it would block politically sensitive terms on its new China site, in agreement with conditions set by Beijing.
US lawmakers are due to meet later this month to discuss the ethical responsibilities of US-based internet companies doing business in China.