Twitter has filed a lawsuit in a US court against five websites that it accuses of creating tools for spamming, as the social media firm battles a wave of automated tweets barraging real users with anything from Viagra ads to virus-ridden links.

Often billed as a service to help a Twitter account gain followers, websites can take control of an account, known as a bot, that follows or sends automated tweets at real users in the hope that some will follow the bot back or click through links the bot has sent out.

'As our engineers continue to combat spammers with strong safeguards and technical efforts, today we're adding another weapon to our arsenal: the law,' Twitter said in a blog post.

Twitter now claims 140 million active users. As its user base has grown, so have bots and spam, emerging as a problem that Twitter fears could dilute the online conversation and irk bona fide users.

Litigation isn't a novel approach: both Facebook and Google have taken spammers to court in recent years with success.

The defendants listed in the suit filed in the U.S. Federal Court in San Franciso are TweetAttacks, TweetAdder, TweetBuddy, James Lucero of justinlover.info and Garland E Harris of troption.com.

The defendants could not immediately be reached for comment by Reuters.

'We've focused on tool providers; They have willfully created tools that enable others to propagate spam on Twitter,' Twitter said.

'With this suit, we're going straight to the source. By shutting down tool providers, we will prevent other spammers from having these services at their disposal. Further, we hope the suit acts as a deterrent to other spammers, demonstrating the strength of our commitment to keep them off Twitter.'

In its blog post, Twitter noted its recent acquisition of Dasient, a web security company that Twitter says represents a part of its 'larger investment in security and fraud protection.' - Reuters