Twitter has stepped up its efforts to purge its membership of terrorism supporters, suspending 125,000 accounts since the middle of 2015, the social network said Friday.

Most of the suspended accounts were linked to the Islamic State terror group, Twitter said in a blog post.

"We condemn the use of Twitter to promote terrorism and the Twitter Rules make it clear that this type of behaviour, or any violent threat, is not permitted on our service," the post said. "As the nature of the terrorist threat has changed, so has our ongoing work in this area."

In recent months Twitter has drawn criticism from officials and security experts for its failure to keep the militant terror organisation from using the microblog service for propaganda and recruitment.

The Washington, DC-based Brookings Institution estimated in 2014 that Islamic State supporters controlled at least 46,000 Twitter accounts.

The data released by Twitter Friday - the first time it has revealed the extent of its shutdown of terror-linked accounts - suggest the group may have had many more.

The San Francisco-based company declared itself "horrified" by extremist violence and said it had increased measures to keep its supporters and promoters off of Twitter, with more staff dedicated to reviews of suspect accounts and spam-fighting programs to help identify them.

But the company pointed out it has a fine line to walk, protecting free speech and privacy without sacrificing security.

"As an open platform for expression, we have always sought to strike a balance between the enforcement of [site rules], the legitimate needs of law enforcement, and the ability of users to share their views freely - including views that some people may disagree with or find offensive," the company said.

Twitter emerged as a unique online tool for organizing real-world action during civil unrest in Iran in 2009-2010 and during the protests of the 2011 Arab Spring, when some protesters used the platform to express dissent and send out web-wide calls for action.

Islamic State has taken a page from that book, using the safe space of social media, especially Twitter and Facebook, to disseminate propaganda and recruit fighters and supporters from around the world.

"Resolve means depriving jihadists of virtual territory, just as we work to deprive them of actual territory," US Democratic presidential candidate Hillary Clinton said in a Brookings speech in December, calling for an "urgent dialogue" between government and tech.

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