SAN FRANCISCO (CBS SF) -- Twitter is warning some users they may have been targets of a cyber-attack from a state-sponsored group.

Last week, notices were sent out to a "small group of accounts," mostly political activists, security researchers in western countries.

Just received this email from Twitter suggesting my account may have been targeted by state-sponsored actors. pic.twitter.com/QLcCzsHphw

— Runa A. Sandvik (@runasand) December 11, 2015

Users were advised to visit the "Tor Project" or the Electronic Frontier Foundation to find anti-surveillance technology and practices.

Here is warning from Twitter to political activists to use #TOR to protect their information. #BigBrother pic.twitter.com/phb4eaQbqY

— Proxy Observer (@proxy_observer) December 14, 2015

Tor is free software that claims to protect users from surveillance by creating a network pathway of encrypted connections. Sources and destinations are harder to trace.