Twitter forced an undisclosed number of users to change their passwords this week after the micro-blogging site discovered it had been invaded by hackers using a complex scheme to gain access to users' information.

"As part of our ongoing efforts to monitor our user base for odd activity, we noticed a sudden surge in followers for a couple of accounts in the last five days. Given the circumstances surrounding this," wrote Twitter director of Trust and Safety Del Harvey, in a blog post Tuesday.

"Given the circumstances surrounding this, we felt it was best to push out a password reset to accounts that were following these suspicious users," said Harvey. He didn't specify exactly how many Twitter users were affected by the problem.

Harvey said the hackers may have gained access to Twitter user names and passwords through third-party torrent sites they created. The torrent sites, which typically allow users to download media files from the Internet, were used to capture log-in details and other information.

Since many individuals use the same user names and passwords for multiple Internet accounts, the hackers were able to apply the captured information to Twitter, and possibly other sites.

"As a general rule, if you signed up for a torrent forum or torrent site built by a third party, you should probably change your password there," said Harvey.

"The takeaway from this is that people are continuing to use the same email address and password (or variant) on multiple sites," Harvey noted. "We strongly suggest that you use different passwords for each service you sign up for," he advised.