Twitter has sent warnings to a number of users that their accounts may have been hacked by "state-sponsored actors".
It is the first time that the social media company has issued such a warning.
Twitter emailed users to say that the hackers may have sought their email or IP addresses, or phone numbers, which it recently began collecting.
The number of accounts affected by the suspected hack is unclear.
Coldhak, a Canadian non-profit organisation, said it had received a warning from Twitter.
"We believe that these actors (possibly associated with a government) may have been trying to obtain information such as email addresses, IP addresses, and/or phone numbers," the email stated.