The accounts frozen since mid-2015 were targeted "for threatening or promoting terrorist acts," said Twitter yesterday, which is under pressure from governments to act.
"Like most people around the world, we are horrified by the atrocities perpetrated by extremist groups," Twitter said on its policy blog.
"We condemn the use of Twitter to promote terrorism and the Twitter rules make it clear that this type of behavior, or any violent threat, is not permitted on our service."
Twitter said it already has rules to discourage this activity but that it was driving up enforcement by boosting staff and using technology to filter violence-promoting content. But it warned there is no easy technological solution.
"As many experts and other companies have noted, there is no 'magic algorithm' for identifying terrorist content on the Internet, so global online platforms are forced to make challenging judgment calls based on very limited information and guidance," Twitter said.