The Syrian Electronic Army (SEA) hacker group appears to have hacked Skype's social media accounts.
Microsoft-owned Skype's Twitter account on Wednesday displayed the message: "Don't use Microsoft emails(hotmail,outlook),They are monitoring your accounts and selling the data to the governments. More details soon #SEA."
It was posted about 10:30GMT but was removed less than two hours later.
Microsoft could not immediately be reached for comment, reported Agence France-Presse (AFP) news agency.
The SEA account belongs to the Syrian Electronic Army, which backs the Damascus government.
In a posting on its own Twitter account, the SEA said "You can thank Microsoft for monitoring your accounts/emails using this details," and listed Microsoft CEO Steve Ballmer's contact information.
The group has previously hacked accounts of The New York Times, AFP and other media organisations.
The SEA's latest attack appears to be linked to documents released by National Security Agency leaker Edward Snowden, which revealed the PRISM surveillance programme.
PRISM is said to give the NSA and FBI easy access to the systems of nine of the world's top Internet companies including Google, Facebook, Microsoft, Apple, Yahoo and Skype.
The programme apparently allowed the NSA to spy on audio and video calls using so-called secret backdoors. Skype has denied the existence of such access.
Tech blogs said that at one point, the Skype blog had the headline: "Hacked by Syrian Electronic Army.. Stop Spying!"
Skype's Twitter account was also said to have posted the message: "Stop spying on people! via Syrian Electronic Army."
But links provided by the SEA's Twitter account to Skype's blog and Facebook account did not work, and the SEA messages appeared to have apparently been removed.