Over the weekend Twitter warned some users that their accounts may have been targeted by state-sponsored hackers.
The alert was the first from the social network and brings it in line with Facebook's and Google's efforts to warn users of attacks on user accounts.
Security researcher Runa Sandvik reported receiving an email from Twitter on Friday advising her that she is one of a "small group of accounts that may have been targeted by state-sponsored actors". Several other security and privacy researchers also reported receiving the email.
Coldhak, a Canada-based non-profit organisation, also received the warning from Twitter but is unaware why it was targeted. Colin Childs, one of Coldhak's founding directors, told Reuters that it hadn't noticed any impact from the attack.
Most communications on Twitter are public. However, the email highlighted that the attackers could have been trying to access account information such as the email addresses, IP addresses, and phone numbers.
Twitter acknowledged that some people who use a pseudonym to tweet may already be concerned by state-sponsored hackers and suggested checking out Tor privacy technology if they are.
However, as Sandvik pointed out, Twitter frequently blocks accounts accessed over Tor. A number of Twitter users in September reported being locked out of their account when accessing it from Tor.
Earlier this year Twitter also reportedly began asking Tor users to provide their phone number to sign up, which came shortly after introducing "new enforcement actions" to tackle accounts that violate its rules. Twitter has denied targeting Tor users with phone-verification requests.
Twitter said it is still investigating the suspected attacks.
Facebook introduced its state-sponsored hacker warning system this October, which arrived one day after Adobe patched a zero-day exploit for Flash that was being used by a suspected Kremlin-backed hacking group Pawn Storm.