Yahoo! has asked a US court to dismiss a lawsuit accusing it of "aiding and abetting" torture in China by releasing information that led to the imprisonment of dissidents.

The search company said yesterday that the legal action was "a political case challenging the Chinese Government" which had no place in the US courts. It said that it had merely been obeying the law when it gave Chinese authorities the registration information of a user who had promoted democracy in a forum.

The company was responding to a lawsuit filed in April by the wife of Wang Xiaoning, a writer with a Yahoo! e-mail account who was jailed for ten years in 2003 after he was found guilty by a Chinese court of "incitement to subvert state power."

Yahoo! was referred to ten times in the court's verdict, and the company has acknowledged handing over information – including to the content of e-mails sent by Mr Wang – when requested to do so.

In a filing with a federal court in San Fransisco, Yahoo! said: "This is a lawsuit by citizens of China imprisoned for using the internet in China to express political views in violation of China law. It is a political case challenging the laws and actions of the Chinese government. It has no place in the American courts."

While Yahoo! "deeply sympathised" with the plaintiffs and their families and "did not condone the suppression of their rights and liberty by their Government," the company said it had "no control over the sovereign Government of the People's Republic of China, the laws it passes and the manner in which it enforces its laws."

The dissidents had "assumed the risk of harm when they chose to use Yahoo! China e-mail and engage in activity they knew violated Chinese law."

Another of the dissidents involved in the case, Shi Tao, was convicted in 2005 of divulging state secrets after he posted online a Chinese Government order forbidding media organisations from marking the anniversary of the Tiananmen Square uprising.

"Free speech rights as we understand them in the United States are not the law in China," Yahoo! said. "Every sovereign nation has a right to regulate speech within its borders."

Legal experts said that the case, which is being brought under the Alien Tort Claims Act and the Torture Victim Protection Act, would face difficulties because of US courts' unwillingness to get involved when foreign nationals fell foul of laws in their own countries.

When the Torture Victim Protection Act was passed in 1992, the first President Bush said that there was a danger that US courts could become embroiled in disputes in other countries. "Potential abuse of the statute would give rise to serious frictions in international relations," he said at the time, and would be "a waste of our own already overburdened judicial resources."

Morton Sklar, executive director of the World Organization for Human Rights USA, which is representing the dissidents, told the New York Times: “It is not the Chinese Government that is the defendant here. It is Yahoo!, for their part in this process. They gave the pieces of information that allowed China to take these actions.”

Several internet companies, including Google, have been criticised for blocking politically sensitive content from their Chinese sites.