Twitter on Tuesday began rolling out a new weapon in the fight against harassment by "trolls" whose often anonymous vitriol can make the messaging service an unwelcoming place.

The move comes as online social networks struggle to balance free speech with intimidation and aggression that make many fearful of speaking freely.

"The amount of abuse, bullying, and harassment we've seen across the internet has risen sharply over the past few years," Twitter said in a statement.

"These behaviors inhibit people from participating on Twitter, or anywhere."

Twitter is expanding a "mute" feature that enables users to block accounts sending inappropriate messages.

Twitter will let users eliminate, or mute, notifications based on keywords, phrases or entire conversations they are not interested in seeing, according to the San Francisco-based company.

"This is a feature we've heard many of you ask for, and we're going to keep listening to make it better and more comprehensive over time," it said.

Twitter policy already prohibits hate spewed based on race, gender, religion, disability or sexual orientation.

- Bearing witness -

Measures announced on Tuesday included providing a more direct way for people to report abusive behavior, even if they are a witness to it and not the target.

"This will improve our ability to process these reports, which helps reduce the burden on the person experiencing the abuse, and helps to strengthen a culture of collective support on Twitter," the service said.

Twitter support staff have been retrained on its policies, including sessions to help understand comments in cultural and historic context, according to the company.

Internal processes have been tuned to deal more effectively with reports of abusive behavior, with a goal of being faster and more transparent in handling situations, Twitter said.

Reasoning for the move included making Twitter a more welcoming platform, ideally ramping up the number of users and the amount of time they spend engaged on the platform.

"Abusive conduct removes the chance to see and share all perspectives around an issue, which we believe is critical to moving us all forward," Twitter said.

"In the worst cases, this type of conduct threatens human dignity, which we should all stand together to protect."

The moves come after a series of complaints and high-profile instances of abuse on the social network.

In July, "Ghostbusters" star Leslie Jones briefly quit Twitter after what she said was a stream of abuse fueled by comments from an editor at the conservative news site Breitbart.

- Instagram as well -

The challenge of making Twitter a safe online venue for open and insightful discourse is believed to be among the reasons the company failed to find a suitor when it courted potential buyers this year.

Twitter said that it has seen a growing trend in people taking advantage of the openness of the service to abuse others.

"We don't expect these announcements to suddenly remove abusive conduct from Twitter," its statement said.

"Instead we commit to rapidly improving Twitter based on everything we observe and learn."

Twitter's expanded mute feature appeared similar to a tool that Facebook-owned Instagram added in September.

The popular photo and video-sharing service began letting users tackle online abuse by creating lists of words that would automatically make comments hidden from sight.

"All different types of people -- from diverse backgrounds, races, genders, sexual orientations, abilities and more -- call Instagram home, but sometimes the comments on their posts can be unkind," Instagram co-founder and chief executive Kevin Systrom said in a blog post at the time.

"To empower each individual, we need to promote a culture where everyone feels safe to be themselves without criticism or harassment."

Instagram also lets users swipe to delete comments, report abusive posted remarks and even have accounts blocked.