Skype owner Microsoft should release information about how much user data it gives to third parties, including government agencies, several organizations and individuals said in a letter to company officials.

The groups, including the Electronic Frontier Foundation, Reporters Without Borders and GreatFire.org, called on Microsoft and Skype to also release information about whether third parties can intercept Skype calls and to explain their position on complying with U.S. wiretap laws. Forty-five groups and more than 60 people signed the letter to Microsoft officials.

With about 600 million users, Skype is effectively one of the largest telecommunications providers worldwide, the letter noted.

"Many of its users rely on Skype for secure communications --- whether they are activists operating in countries governed by authoritarian regimes, journalists communicating with sensitive sources, or users who wish to talk privately in confidence with business associates, family, or friends," the letter said.

Skype is reviewing the letter, a spokesman said.

Skype users "work in the face of persistently unclear and confusing statements about the confidentiality of Skype conversations," including access by government agencies, the letter said.

The letter called on Microsoft and Skype to disclose what user information they collect and to disclose Skype's relationship with Chinese mobile provider TOM Online and other third-party licensees of Skype technology.

Other online companies, including Google and Twitter, release regular reports detailing request for user data by third parties, the letter noted.