Pro-Tibet activists say they are targets of sophisticated cyber attacks designed to disrupt their work.
Some groups serving as channels for information on events in Tibet say they have received a large volume of emails infected with computer viruses in the past few days.
Lhadon Tsering, executive director of the New York-based Students for a Free Tibet, says that her organization has been targeted.
The source of the attacks is unknown.
In a related development, a group that has been pressuring China to use its influence to end violence in Sudan's Darfur region says hackers in China have accessed its computer systems.
The U.S. Federal Bureau of Investigation is looking into the claims by the Save Darfur Coalition. "The Washington Post" newspaper says a senior Chinese official, who did not want to be identified, denied the allegations.
The Save Darfur Coalition has been critical of China for not using its influence with the Sudanese government to end the conflict in Darfur. China is Sudan's largest trading partner and a major supplier of arms to the country.